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Surname, Initial(s). (2012) Title of the thesis or dissertation. PhD. (Chemistry)/ M.Sc. (Physics)/ M.A. (Philosophy)/M.Com. (Finance) etc. [Unpublished]: University of Johannesburg. Retrieved from: https://ujdigispace.uj.ac.za (Accessed: Date). A3 to C~IC R.

A SYNTHESIS OF THE STRUCTURAL GEOLOGY OF THE NORTHERN TRANSVAAL

by

CARLOS JORGE CHERNICOFF

Dissertation

submitted in fulfilment of the requirements

for the degree of

MASTER OF SCIENCE

in

GEOLOGY

FACULTY OF SCIENCE

at the

RAND AFRIKAANS UNIVERSITY

Supervisor Prof. C. Roering Co-supervisor Prof. D.O. Van Reenen

NOVEMBER 1984 - SUt·1MARY -

The study area is subdivided into seven tectonic domains, viz. Bandelierkop, Southern Beit Bridge Complex, Alldays, Hestern Transvaal, Soutpansberg, Waterberg and Lebombo domains.

The Bandelierkop domain comprises the Southern Marginal Zone of the Limpopo Belt (Bdl) and the northern portion of the Kaapvaal craton (Bd2). Bdl is characterized by the presence of highly disrupted xenoliths of pelitic and mafic supracrustal rocks distributed in a "sea " of granitic ·material. This contrasts with the much bigger supracrustal xenoliths in Bd2, i.e. the greenstone belt relicts.

The granulite facies rocks of Bdl may have been upthrust roughly from south to north along one or more south-dip­ ping thrust faults soling into a gently-dipping to flat­ lying basal shear zone. Since this model reveals the e­ xistence of the Kaapvaal craton partly on edge, the pro­ gressively higher grade of regional metamorphism (from greenschist to granulite facies) encountered in the Bande­ lierkop domain, from south (Bd2) to north (Bdl), could express the transition from upper crust to lower crust as exposed on the present land surface.

The structural trends found in Bdl and Bd2 are not strictly confined to either area, and some overlapping exists. The most distinct structural trend in Bdl is a west-north­ west fold trend, a northeast fold trend also being recog­ nized in the eastern portion of Bdl. Bd2 exhibits f~ld trends varying from west-northwest to northeast in orienta­ tion; in this area there is ample evidence for the later nature of the northwest- to west-northwest oriented folding event. The east-northeast- to northeast folding is con­ ~dered to represent an event independent of the effect of deformation of the Kudus River shear zone to which this folding was previously thought to be related. The portion of the Central Zone of the Limpopo Belt falling within South African territory comprises the Alldays and Southern Beit Br_idge Compl ex (SBBCd) doma tn s., There e­ xists a marked contrast between the roughly north trending fold trends of the Alldays domain and the east-northeast fold structure of the southernmost part of the Central Zone, the SBBCd. The relatively gentle plunges of the fold structure of the Alldays domain may have accentuated the size of the regional folds in this region. The SBBCd occupies a narrow east-northeast oriented strip extending immediately south of the Alldays domain, where tightly folded gneisses and metasedim~nts are exposed. all strata are strongly aligned in the regionally extensive east­ northeast direction. A contrasting difference between the SBBCd and the surrounding Precambrian terranes is clear, as the former domain conveys a relatively higher strain; this evidence of heterogeneous strain suggests that the SBBCd represents a shear zone. The shearing movement may have been of thrust type, consistent with the thrust model re­ ferred to above, and the SBBCd may be regarded as the thrust plane. In'iiew of the steeply-dipping, south-southwest oriented fold axes known to exist along part of the SBBCd (area south of Messina), it would appear that, at least in that area, the thrust movement would have been towards the north-northeast. A late, involved history of deformation followed in the SBBCd, during which one or more episodes of wrench-type movement may have taken place.

The Soutpansberg domain is a relatively narrow and long fault zone of Proterozoic to Phanerozoic age that separates 'mobile belt' environment to the north, from 'cratonic' environment to the south. A linked fault system character­ ized by south-dipping, normal listric faults soling into ? gently-dipping major detachment surface may account for the structural pattern of this domain. The thrust system referred to above, which developed earlier in the geological ./ history of the region along the Limpopo Belt - Kaapvaal craton boundary, may have controlled the late extensional displacement in the Soutpansberg domain. The overall structure of the Waterberg domain would seem to be controlled by the superimposition of northeast- and northwest oriented gentle folds and, with the exception of locally intense deformation in the early Waterberg basin, would appear to have developed on a relatively stable portion of crust.

The structure of the Lebombo domain is characterized by a north-south striking monocline with gentle east dip.

Most of the lineaments inferred from the aeromagnetic sur­ vey of the study area originate from dyke intrusions. Faulting, fracturing and shearing are less clearly mani­ fested in the pattern of the aeromagnetic contour maps, instead they are better correlated with the lineaments inferre~ from LANDSAT imagery and from the drainage patterns of the region. Opsomming

Die studiegebied word onderverdeel in sewe tektoniese domeine, naamlik die Bandelierkop-, Suidelike Beitbrug­ kompleks, Alldays-, Westelike Transvaal-, Soutpansberg-, Waterberg- en lebombodomeine.

Die Bandelierkopdomein sluit ~·ie suidelike randsone van die Limpopogordel (Bdl) en die noordelike gedeelte van die Kaapvaalkraton (Bd2) in. Bdl word gekenmerk deur die teenwoordigheid van uiters uiteengeskeurde xeno­ liete van peliti~se en mafiese deklaag gesteentes, d.w.s groensteengordeloorblyfsels, in Bd2.

Die granulietfasies gesteentes van Bdl kon opgehef gewees het van suid na noord langs een of meer suidhel1ende listriese stootverskuiwings wat oorgaan in 'n vlakhel­ lende basale skuifskeursone. Aangesien hierdie model die hellende voorkoms van die Kaapvaalkraton aandui, kan die progressiewe ho~rgraad van regionale metamorfose (van groenskis tot granulietfasies) wat in die Bandelier­ kopdomein aangetref word die oorgang van die vlakker tot die dieper kors aandui soos blootgele op die teen­ woordige landoppervlakte.

Die struktuurneigings wat in Bdl en Bd2 aangetref word, is nie streng tot die een of die ander gebied beperk nie, maar daar is 'n mate van oorvleueling. Die duidelikste struktuurneiging in Bdl is 'n WNW plooineiging, terwyl 'n NO plooineiging in die oostelike gedeelte van BDl op­ gemerk word. Bd2 vertoon plooineigings wat wissel van WNW tot NO in ori~ntasie; in hierdie gebied is daar vol­ doende bewys vir die later aard van die NW tot WNW ge­ ori~nteerde plooiingsverskynsels. Die aNa tot NO plooi­ ing word beskou as In verskynsel wat onafhanklik is van die vervormingseffek van die Koedoesrivierskuifskeursone. Die gedeelte van die Sentrale Sone van die Limpopo­ gordel binne Suid-Afrikaanse gebied sluit die Alldays­ en die Suidelike Beitbrugkompleksd6meine (SGBCd) in, Daar bestaan In duidelike kontras tussen die noordstrek­ kende plooineigings van die Alldaysdomein en die ONO plooistrukture van diemees suidelike gedeelte van die Sentrale Sone -- die SBBCd. Die relatiewe vlak duiking van die Alldaysdomein-plooistrukture kan die grootte van die regionale plooie in hierdie gebied beklemtoon. Die SBBCd beslaan In smal ONO-geori~nteerde strook on­ middelik suid van die A1ldaysdomein waar sterkgep100ide gneise en metasedimente blootgele word; al die strata is sterk gerig in die regionale OND rigting. In Kontras­ terende verskil tussen die SBBCd en die omliggende voor­ Kambriese gebiede is ooglopend aangesien die eersgenoem­ de domein In relatief ho~r mate van vervorming aantoon. Hierdie bewys vir onge1yksoo~tige vervorming wi1 daar- op dui dat die SBBCd In skuifskeursone verteenwoordig. Die sKuifskeurbeweging kon 'n tipe stootversKuiwing ge­ wees het in ooreenstemming met die stootverskuiwings­ model waarna hierbo verwys is, en die SBBCd kan as die stootverskuiwingsvlak beskou word.

In die 1ig van die stylhellende SSW-geori~nteerde plooi­ asse, wil dit voorkom asof die stootverskuiwingsrigting in daardie gebied na die NNO was. In laat en ingewikkel­ de vervormingsgeskiedenis het in die SBBCd gevolg in die loop waarvan een of meer episodes van verwringing kon plaasgevind het.

Die Soutpansbergdomein is 'n re1atief smal en lang ver­ skuiwingsone van die Proteroso~ese tot die Phaneroso~ese Tye wat die mobiele gordel-omgewing in die noorde van die kr a t onies e omgewing in die suide skei. 'n Gekoppelde verskuiwingsisteem wat gekenmerk word deur suidhellende listriese afskuiwings wat in In geleidelik-hellende hoof­ verp1asingssone oorgaan, kan In verklaring bied vir die struktuurpatroon van hierdie domein. Die stootYer­ skuiwingsisteem waarna hierboverwys is en wat vroe~r in die geologiese geskiedenis van die gebied langs die Limpopogordel-Kaapvaalkratongrens ontwikkel het, kan die laat vervorming onder verspanning in die Soutpansberg ge- bied verklaar.

Die algemene struktuur van die Waterbergdomein word klaarblyk1ik beheer deur die interaksie van oop golwen­ de plooie in 'n noordoos en noordweste1ike rigting en met uitsondering van die loka1e-intensiewe vervorming in die vroe~ Waterbergkom wi1_dit voorkom asof die de­ formasie 'n relatief stabiele korsgedeelte ontwikkel het.

Die struktuur van die Lebombodomein word gekenmerk deur 'n noord-suid strekkende monoklien met 'n vlakwaartse hel1ing.

Die meeste van die lineamente in die studiegebied wat afgelei is van die 1ugmagnetiese opnames het waarskyn­ 1ik onstaan deur gangintrusies. Verskuiwing, breukvorm­ ing en skuifskeuring is minder duide1ik geopenbaar in die patroon van die 1ugmagnetiese kontoerkaarte, aange­ sien hullebeter gekorreleer is met die 1ineamente af­ ge1ei van Landsat-bee1de en van die dreineerpatroon van die landstreek. ACKNOWLEDGMENTS

Thanks are due to the Council for Scientific and Industrial Research for the financial support received.

I am greatly indebted to Prof. C. Roering, without whose expert advice and assistance this research would not have been completed. lowe a debt of gratitude to Mrs. J.H. Legg, who kindly drafted the regional map as well as the outlines of the several small-scale maps included in this thesis. CONTENTS Page 'fl. INTRODUCTION 1 1.1. Location of study area 1 1.2. Previous work 2 1.3. Scope of study 3

~2. GEOLOGICAL SETTING OF THE STUDY AREA 6 , 2.1. General statement 6 2.2. Archaean basement 6 2.2.1. Limpopo Belt 6 2.2.2. Granite-greenstone terrane 12 2.3. Proterozoic to Phanerozoic cover sequences 14 2.3.1. HaterbergGroup 15 2.3.2. Soutpansberg Group 16 2.3.3. Karoo Sequence 16 2.3.4. Cretaceous to Recent 17

0( 3. STRUCTURE 18 3.1. Introduction 18 3.2. Domains of the Archaean basement 19 3.2.1. Bandelierkop domain 19 3.2.1.1. General statement 19 3.2.1.2. General description 19 3.2.1.2.1. Area 1 19 3.2.1.2.2. Area 2 22 3.2.1.3. Faults, fractures and shear zones 25 3°.2.1.3.1. General statement 25 3.2.1.3.2. Interpretation 26 3.2.1.4. Dykes 28 3.2.1.4.1. General statement 28 3.2.1.4.2. Interpretation 29 3.2.1.5. Discussion 31 3.2.1.5.1. Areal 31 3.2.1 .5.2. Area 2 36 3.2.2. Southern Beit Bridge Complex domain 42 3.2.2.1. General statement 42 3.2.2.2. Main structural features 42 3.2.2.2.1. General description 42 Page 3.2.2.2.2. Faults and fractures 44 3 • 2 •2 • 2 .2 . 1 • General statement 44 3.2.2.2.2.2. Interpretation 45 3 •2. 2 . 2 • 3. Dykes _ 46 3.2.2.2.3.1. General statement 46 3.2.2.2.3.2. Interpretation 47 3.2.2.3. Interpretation of main structural features 48 3.2.2.3.1. Earlier workers 48. 3.2.2.3.2. Discussion 50 3.2.3. Alldays domain 55 3.2.3.1. General statement 55 3.2.3.2. ~1ain structural features 55 3.2.3.2.1. General description- 55 3.2.3.2.2. Faults and fractures 58 3.2.3.2.2.1. General statement 58 3.2.3.2.2.2. Interpretation 58 3.2.3.2.3. Dykes 60 3.2.3.2.3.1. General statement 60 3.2.3.2.3.2. Interpretation 61 3.2.3.3. Interpretation of main s~ructural features 62 3 • 2 . 3 . 3 •1. Ear 1 i e r w0 r ke r s 62 3.2.3.3.2. Discussion 65 3.2.4. Hestern Transvaal domain 67 3 . 2 . 4 . 1 • Genera 1 s tatement 67 3.2.4.2. Fractures and faults 67 3.2.4.2.1. General statement 67 3.2.4.2.2. Interpretation 68 3.2.4.3. Dykes 70 3.2.4.3.1. General statement 70 3.2.4.3.2. Interpretation 70 3.3. Domains of the Proterozoic to Phanerozoic 75 cover sequences 3 . 3 . 1. Sou t pan s be r g do rna i n 75 3.3.1.1. Introduction 75 3 . 3 . 1 • 2. Fau1t sand f r a c tu res 75 3.3.1.2.1. General statement 75 3.3.1.2.2. Interpretation 76 Page 3.3.1.3. Dykes 77 3.3.1.3.1. General statement 77 3.3.1.3.2. Interpretation 78 3.3.1.4. Discuss-ion 79 3.3.2. Waterberg domain 87 3.3.2.1. General statement 87 3.3.2.2. General description 87 3.3.2.3. Faults and fractures 89 3.3.2.3.1. General statement 89 3.3.2.3.2. Interpretation 90 3.3.2.4. Dykes 91 3.3.2.4.1. General statement 91 3.3.2.4.2. Interpretation 91 3.3.3. Lebombo domain 96 3.3.3.1. General description 96 3.3.3.2. Faults and fractures 97 3.3.3.2.1. General statement 97 3.3.3.2.2. Interpretation 97 3.3.3.3. Dykes 97 3.3.3.3.1. General statement 97 3.3.3.3.2. Interpretation 98

4. AEROMAGNETIC TRENDS OF THE STUDY AREA 102 4.1. Introduction 102 4.2. Description 106 4.2.1. Bandelierkop domain 106 4.2.2. Southern Beit Bridge Complex domain - 109 Al1days domain region 4.2.3. Western Transvaal domain 110 4.2.4. Soutpansberg domain 114 4.2.5. Haterberg domain 115 4.2.6. Lebombo domain 119 4.3. Interpretation 121

5. STRUCTURE AND UNDIFFERENTIATED LINEAMENTS 122 INFERRED FROM LANDSAT IMAGERY AND FROM THE DRAINAGE PATTERNS OF THE STUDY AREA 5.1. Introduction 122 5.2. Description 123 Page 5.2.1. Bande1ierkop domain 123 5.2.2. A11days domain 127 5.2.3. Southern Beit Bridge Complex domain 128 5.2.4. Western ~ransvaa1 domain 132 : 5.2.5. Soutpansberg domain 132 5.2.6. Waterberg domain 137 5.2.7. Lebombo domain 141 5.3. Interpretation 144

6. SYNOPSIS AND CONCLUSIONS 145 - References 149 LIST OF ILLUSTRATIONS Page Fig. 1. Locality map 1 Fig. 2. Geological setting of the study area 7 Fig. 3. Tectonic subdivision of the northern Transvaal 18 Fig. 4. Schmidt equal area stereographic projections of 21 foliations and lineations (IISand River domain") Fig. 5. Schmidt equal area stereographic projections of 24 fol iations and 1ineations ("01 ifantshoek doma in") Fig. 6. Rose diagram of faults, fractures and shear zones ­ 27 Bande1ierkop domain Fig. 7. Rose diagram of dykes - Bande1ierkop domain 30 Fig. 8. (a): Gravity pattern over the exposed portion of the 32 . Limpopo Bel t (b): Average gravity profile across the eastern portion 33 of the Limpopo Belt Fig. 9. Schematic secti~n across the Southern Marginal Zone of 35 the Limpopo Belt Cal, and across the Soutpansberg fault zone (b) Fig. 10. Locality map - Bandelierkop domain 39 Fig. 11. Map of faults, fractures and shear zones - Bandelierkop 40 domain Fig. 12. Map of dykes - Bandelierkop domain 41 Fig. 13. A schematic cross-section of the Southern Beit Bridge 44 Complex domain Fig. 14. Rose diagram of faults and fractures - Southern Beit 46 Bridge Complex domain Fig. 15. Rose diagram of dykes - Southern Beit Bridge Complex 47 domain Fig. 16. Locality map - A11days and Southern Beit Bridge Complex 52 domains Fig. 17. Map of faults and fractures - A11days and Southern Bei~ 53 Bridge Complex domains Fig. 18. Map of dykes - Alldays and Southern Beit Bridge Complex 54 domains Fig. 19. Photogeo1ogical interpretation of the Messina district 57 Fig. 20. Rose diagram of faults and fractures - Al1days domain 59 Fig. 21. Rose diagram of dykes - Alldays domain 61 Page Fig. 22. East-northeast shear couple proposed by Bahnemann (l972) 63 to explain the fold structure of the "Cross-folded Belt" (a), and structural pattern resulting from shear deforma- tion (b) Fig. 23. Rose diagram of fractures and faults - Western Transvaal 68 domain Fig. 24. Rose diagram of faults - Western Transvaal domain 69 Fig. 25. Rose diagram of dykes - Western Transvaal domain 71 Fig. 26. Locality Map - Western Transvaal domain 72 Fig. 27. Map of fractures and faults - Western Transvaal domain 73 Fig. 28. Map of dykes - Western Transvaal domain 74 Fig. 29. Rose diagram of faults and fractures - Soutpansberg domain 77 Fig. 30. Rose diagram of dykes - So~tpansberg domain 79 Fig. 31. Cross-sectional sketches showing some depositional and 82 rotational characteristics of strata that are displaced by listric faults Fig. 32. Locality map - Soutpansberg domain 84 Fig. 33. Map of faults and fractures - Soutpansberg domain 85 Fig. 34. Map of dykes - Soutpansberg domain 86 Fig. 35. Rose diagram of faults and fractures - Waterberg domain 90 Fig. 36. Rose diagram of dykes - Waterberg domain 92 Fig. 37. Localitymap - Waterberg domain 93 Fig. 38. Map of fa~lts and fractures - Waterberg domain 94 Fig. 39. Map of dykes - Waterberg domain 95 Fig. 40. locality map - lebombo domain 99 Fig. 4l. Map of faults and fractures - lebombo domain 100 Fig. 42. Map of dykes - Lebombo domain 101 Fig. 43. Examples of magnetic regimes of low (a), intermediate 104 to high {b}, and very high (c) intensities Fig. 44. Examples of lineaments inferred from the aeromagnetic 105 maps Fig. 45. Strike orientations of magnetic lineaments - Bande1ierkop 107 domain Fig. 46. Map of magnetic lineaments - Bande1ierkop domain 108 Fig. 47. Strike orientations of magnetic lineaments - Alldays and 110 Southern Beit Bridge Complex domains Fig. 48. Map of magnetic lineaments - Alldays and Southern Beit 111 Bridge Complex domains Fig. 49. Strike orientations of magnetic lineaments - Western 112 Transvaal domain Page Fig. 50. Map.of magnetic lineaments - Western Transvaal domain 113 Fig. 5l. Strike orientations of magnetic lineaments - Sout~· 115 pansberg domain Fig. 52. Map of magnetic lineaments - Soutpansberg domain 116 Fig. 53. Strike orientations of magnetic lineaments - Water- 117 berg domain Fig. 54. Map of magnetic lineaments - Waterberg domain 118 Fig. 55. Map of magnetic lineaments - Lebombo domain 120 Fig. 56. Rose diagram of LANDSAT lineaments - Bandelierkop 124 domain Fig. 57. Map of LANDSAT lineaments - Bandelierkop domain 125 Fig. 58. Map of drainage lineaments - Bandelierkop domain 126 Fig. 59. Rose diagram of drainage lineaments - Bandelierkop 127 domain Fig. 60. Rose diagram of drainage lineaments - Alldays domain 128 Fig. 6l. Rose diagram of LANDSAT lineaments - Southern Beit 129 Bridge Complex domain (eastern portion) Fig. 62. Map of drainage lineaments - A11days and Southern 130 Beit Bridge Complex domains Fig. 63. Map of LANDSAT lineaments - Southern Beit Bridge 131 Complex domain (eastern portion) Fig. 64. Map of drainage lineaments - Western Transvaal domain 133 Fig. 65. Rose diagram of LANDSAT lineaments - Soutpansberg domain 134 Fig. 66. Rose diagram of drainage lineaments - Soutpansberg 134 domain Fig. 67. Map of LANDSAT lineaments - Soutpansberg domain 135 Fig. 68. Map of drainage lineaments - Soutpansberg domain 136 Fig. 69. Rose diagram of LANDSAT lineaments - Waterberg domain 138 Fig. 70. Rose diagram of drainage lineament~ - Water berg domain 138 Fig. 71- Map of LANDSAT lineaments - Waterberg domain 139 Fig. 72. Map of drainage lineaments - Waterberg domain 140 Fig. 73. Rose diagram of LANDSAT lineaments - Lebombo domain 141 Fig. 74. Map of LANDSAT lineaments - Lebombo domain 142 Fig. 75. Map of drainage lineaments - Lebombo domain 143

Map 1. A structural map of the northern Transvaal (scale 1:500 000) 1

1. Introduction 1.1. Location of study area

The study area is situated in the northern part of the Transvaal Pro~ince in the Republic of South Africa, between approximately 22°00'5 and 24°00'5, and 28°00 'E and 32°00 'E (A11days, Messina, Pietersburg and part of the Tzaneen sheets); and between 23°00'5 and 25°00 15, and 26°00 'E and approximately 29°00 'E (E11isras, and part of the Thabazimbi and Ny1stroom sheets)(figure 1).

It covers part of the Central Zone, and the Southern Marginal Zone of the Limpopo Belt, as well as portions of the northern (north of the Murchison Range) and western Kaapvaa1 craton (refer 2., Geological setting of the study area).

20'[ )0'[

Figure 1. Locality map. Study area 2

1.2. Previous work

A key to the previous geological investigation in the various structural domains into which the study area has been subdivided is briefly given below.

Extensive research has been carried out in the Limpopo Belt by numerous workers. Cox et al. (1965) first re­ cognized a large-scale tectonic zoning within the Limpopo Belt; Van Breemen (1968; cited by Van Breemen, 1970) undertook the first systematic geochronological study; and the first comprehensive geological descrip­ tion derives from Mason (1973). The Messina district (Central Zone of the Limpopo Belt) was mapped by SOhnge (1944) and described by the same author in a subsequent memoir of the Geological Survey (1945) and by SOhnge et a1. (1948). Bahnemann (1972) and Horrocks (1981) completed structural and metamorphic studies in the same area. Geochronological studies in the Central Zone were undertaken by Barton et a1. (1977), Barton (1979), Barton et al. (1979a) and Barton et al. (1983). McCourt (1983) carried out comprehensive geological and structural sbElies in the Palala shear zone which probably marks the boundary between the Limpopo Belt and the Kaapvaa1 craton in the Koedoesrand area.

Reconnaissance mapping of the Southern Marginal Zone of the limpopo Belt was undertaken by Du Toit and Van Reenen (1977); structural studies, by Du Toit (1979); metamorphic stu~ies, by Van Reenen and Du Toit (1977), Van Reenen (1983) and also by Du Toit (1979). A comprehensive geochronological study of this area derives from Barton et a1.(1983). 3

Geological work in the portion of the Kaapvaal craton dealt with in this study was carried out by Grobler (1972) in the Pietersburg greenstone belt and by Prinsloo (197]) in the Sutherland greenstone-granite terrane.

The geology of the Soutpansberg region was surveyed by Van Eede~ etal. (1955). Comprehensive geological and structural studies were carried out by Jansen (1975) and Barker (1979).

The geology of the Waterberg basins is summarized in a recent memoir of the Geological Survey (Jansen, 1982).

Bristow (1976, 1980 and 1982) has carried out extensive geological studies in the lebombo region.

1.3. Scope~f study

This thesis was conceived as a study of the broad structural features of the predominantly Precambrian terranes of the northern Transvaal, mainly the north­ ernmost part of .the Kaapvaal craton (including the Southern Marginal Zone of the limpopo Belt) and the portion of the Central Zone of the limpopo Belt falling within South African territory, as well as the Sout­ pansberg trough that separates these two areas. The study thus lead the author to focus attention on the, early crustal evolution of the region and the relation­ ship between early and late episodes of deformation.

In order to realize this objective, the thesis first involved the preparation of a regional structural map 4 of the northern Transvaal, as commissioned by the Working Group for the compilation of a Tectonic Map of South Africa, sponsored by the National Geoscience Programme of the C.S.I.R. (Council for Scientific and Industrial Research).

In compiling this map, the author analyzed all previous investigations carried out in the region, including small-scale geological maps from theses written about the area, large-scale maps of the Geological Survey and further literature referring to the area, from which the author obtained measurements of strike and dip of strata, faults, foliations and lineations, where available. On the basis of this information, structural form lines were traced, but are unevenly distributed throughout the study area due to insufficient informa­ tion in certain sections.

Furthermore, the author makes proposals concerning the structural evol.ution of the region. These are synthe­ sized in the form of a schematic theoretical section across part of the eastern segment of the study area.

In addition, an examination of the aeromagnetic survey of the northern Transvaal was carried out. The regional pattern of lineaments and magnetic intensity regimes was interpreted in the light of the geological and structural background of the area. Similarly, lineaments inferred from LANDSAT imagery and drainage patterns (the latter derived from the relevant topo­ graphic and geological maps) of the study area were also compared to the overall structure and the magnet­ ic lineaments.

From the above mentioned, it is clear that this thesis differs from others in its approach in that neither 5 field work nor microscopic-chemical analyses were carried out; instead, the author's work was largely concentrated on the examination of previously written structural data of the area, the analysis, and finally the interpretation of such data. This information is to be used for the new Tectonic Map of South Africa. 6

2. Geological setting of study area 2.1. General statement

This chapter is intended to give a brief summary of the main geological units present in the study area, viz. the Limpopo Belt, the granite-greenstone terrane and the Proterozoic to Phanerozoic cover sequences.

2.2. Archaean basement 2.2.1. Limpopo Belt

The Limpopo Belt (Watkeys, 1983; and figure 2) is defined as an approximately linear east-northeast trending zone of Precambrian rocks which have under­ gone high-grade metamorphism and polyphase deformation. The Belt is situated between the Rhodesian craton in the north and the Kaapvaal craton in the south, and is approximately 500 km in length and 250 km in width. The northern and southern margins are marked byortho­ pyroxene isograds signifying the first appearance of granulite-facies rocks (Robertson, 1968; Du Toit and Van Reenen, 1977). In the east it is terminated by the younger Mozambique Belt, and in the west it is thought to end immediately west of 27°30'E (Key and Hutton, 1976).

A zonation has been recognized within the Belt (fig- , ure 2), where Northern and Southern Marginal Zones, with predominant east-northeast trending structures, are separated 'by a Central Zone of approximately northerly trending structures. The Northern and South­ ern Marginal Zones comprise high-grade metamorphic / 7

20"

o 50 100 150 I , , ,

I~I SHEAR ZONE~ REACTIVATED SINCE 2000 Ma

~POST~2600 Ma SHEAR ZONES I..;.:;] ORTHOPYROXENE ISOGRADS"· AND ORTHOAMPHIBOLE ISOGRAD­ ~ PROTEROZOIC TO PHANEROZOIC COVER SEQUENCES 1++1 -2600 Ma GRANITIC PLUTONISM c=J CENTRAL ZONE - LIMPOPO BELT NS01 GRANULITE GRADE MARGINAL ZONES - LIMPOPO BELT ~ GRANITE-GREENSTONE TERRANES ~ INTERNATIONAL BOUNDARIES

Figure 2. Geological setting of study area. (after Du Toit et a1., 1983) 8

equivalents of the flanking Archaean granite~green" stone terranes,whereas the Central Zone is dominated by high-grade paragneisses and metasediments, includ­ ing layered igneous complexes but rare intrusive granites (Watkeys, 1983).

The Central Zone of the Limpopo Belt (figure 2) is underlain by a variety of ortho- and paragneisses, including the Sand River Gneisses which are considered to represent the basement to the more abundant supra­ crustal gneisses of the Beit Bridge Complex. The Sand River Gneisses comprise a variety of grey, leucocratie hypersthene-bearing gneisses of quartz-dioritic and granodioritic compositions, which together with deform­ ed and metamorphosed ancient dykes and veins of various ages and compositions intruded into the gneisses, are referred to as the Sand River Migmatite Complex (Fripp, 1983). The Sand River Gneisses have yielded a Rb-Sr whole-rockjsochron of about 3800 Ma for a metamorphism which is regarded as the fabric forming event in these rocks (Barton et al., 1977). Two suites of ancient tholeiitic dyke intrusion have been recognized in the Sand River Migmatite Complex (Barton et al., 1977): the oldest has been dated at about 3570 Ma by Rb-Sr whole-rock techniques and intrudes the Sand River Gneisses; and the younger dyke has given an age of about 3060 Ma (Rb-Sr whole-rock isochron) and intrudes the Sand River Gneisses, the Beit Bridge Complex and the Messina Layered Intrusion.

The ~ominant lithologies of the Beit Bridge Complex include a variety of quartzo-feldspathic gneisses with numerous intercalations and lenses of other para­ gneisses which have varying proportions of amphibole, pyroxene, mica, garnet, cordierite and sillimanite. Quartzite, banded magnetite quartzite, pyroxenitic 9

amphibolite, calc-silicate gneiss and marble also occur. A distinctive granitoid known as the Singelele Gneiss is intercalated with other supracrustal litho­ logies. TheSingelele Gneiss has given an age of about 2600 Ma (Rb-Sr whole-rock isochron; Barton et al., 1979a).

An anorthosite-bearing basic layered intrusion, the Messina Layered Intrusion (Barton et al., 1979b; refer­ red to as Messina Suite by S.A.C.S., 1980), has been emplaced at the contact between the basement complex (Sand River Gneisses) and the overlying supracrustal succession {Beit Bridge:Complex),,·and also within the ~upracrustal succession itself. Anorthosite and leuco- gabbro, the predominant rock types present, are inter­ calated with lesser amounts of gabbro, melagabbro and ultramafic rocks •. In general, all rock types compris­ ing the Messina Layered Intrusion have been extensive­ ly but variably deformed and recrystallized into gneisses. Locally, however, evidence for an original igneous nature of these rocks is preserved (Barton et al., 1979b). The. Messina Layered Intrusion has a Pb-Pb isochron of about 3270 Ma (Barton, 1983a), although the Rb-Sr whole-rock isochron for these rocks gives a date of about 3150 Ma, which is probably a metamorphic age (Barton et al., 1979b). This 3150 Ma age is thus a pervasive event and probably represents the time of the peak high-grade metamorphism. This suggests that the supracrustal rocks (Beit Bridge Com­ plex) were deposited sometime between 3570 Ma (the' age yielded by the oldest of the ancient tholeiitic dyke intrusions only recognized in the Sand River Gneisses) and 3270 Ma ago.

The area northwest of Messina is underlain by a distinctive granitoid characterized by feldspar mega- 1 0 crysts, which is known as the Bulai Gneiss ~Dhnge, 1945). It is generally considered to have been in­ truded into the basement rocks partly syntectonically about 2700 Ma ago (Barton et al., 1979a; Fripp, 1983; and Hatkeys et al., 1983).

The Southern Marginal Zone of the Limpopo Belt (figure 2) consists of extremely deformed ortho- and para­ gneisses at the granulite and upper amphibolite grades of regional metamorphism. The rock types are sub­ divided into a basement mig~atitic tonalite and trondhjemite gneiss, termed the Baviaanskloof Gneiss, and the overlying supracrustal Bandelierkop Formation which is probably of greenstone origin (Du Toit and Van Reenen, 1977).

This Southern Zone comprises two high-grade metamorphic zones successively defined by the presence of ortho­ amphibole and hypersthene in the pelitic gneiss of the Bande1ierkop Formation. In the north it is bounded by the Soutpansberg mountains, whereas the southern boundary is defined by the southern limit of the ortho­ amphibole zone. It. has therefore been suggested that a metamorphic transition exists between the Southern Marginal Zone and the granite-greenstone terrane of the Kaapvaa1 craton (Du Toit and Van Reenen, 1977).

According to Du Toit et al. (1983), the Baviaanskloof Gneiss comprises greyish migmatitic tona1itic and trondhjemitic gneisses, mainly composed of quartz, ­ plagioclase, hornblende (or biotite), usually well banded. Various degrees of anatexis"have caused internal differences within the Baviaanskloof Gneiss, i.e. from very well banded gneiss in which some cross­ cutting leucocratic veins occur, to a virtually totally homogenized rock in which only vestigial biotite trails 11

remain suggesting the pre-existence of melasomes.

The Bandelierkop Formation is subdivided into three members on the basis of their distinctive lithological variations, namely a lower Ultramafic Member, a middle Mafic Member and an upper Pelitic Member. The Ultra­ mafic Member consists of massive black to dark green coarse-textured rocks (peridotite, pyroxenite, dunite and hornblendite) occurring as a series of discon­ tinuous lens- and pod-like bodies which are dispersed in the Baviaans~loof Gneiss: The Mafic Member contains, presumably at the basal and upper contacts respectively, lenses and layers of ultramafic and pelitic material while thin beds of banded iron­ formation occur sporadically throughout the succession. It is distributed throughout the area as variably sized and shaped remnants of a previously more exten­ sive succession. The rocks of the Pelitic Member are characterized by a foliation which varies from fine banding, to a gne iss i c t extur e whi chisenhanced by white granitic 1entic1es and granitic segregations and thus gives rise to a migmatized character. lenses and beds of banded iron-formation and banded magnetite quartzite are commonly associated with the Pelitic Member.

Repeated granite plutonism is conspicuous in the South­ ern Marginal Zone. It can be shortly summarized as follows: The 2460 Ma old On1ust Gneiss Pluton (Barton et a1 ., 1983) comprises a finely banded greyish gneiss phase and an even lighter-coloured, poorly foliated granitic phase; the 2600 Ma old Matok Pluton (Barton et al., 1983) is composed of an older mafic phase and a younger granitic pha~e; and the 2456 Ma old Palmiet­ fontein Granite Plutons are light brown to grey in 12

in colour, medium-grained with local pegmatite phases, being haphazardly scattered throughout the high-grade terrane, and cutting the rocks of the Baviaanskloof Gneiss and the Bandelierkop Formation discordantly. The Palmietfontein Granite Plutons are unaffected by the Limpopo Belt deforming events, therefore indicat­ ing the end of all major tectono-metamorphic activity in the Southern Marginal Zone. In addition, the Schiel Alkaline Complex is an irregularly-shaped body (refer 3.2.1.2.2.) comprised mainly of syenite, quartz syenite and alkali granite.- The isotopical data for the Schiel Complex are equivocal (Barton et al., 1983), and it has therefore been suggested that it is a late event related to the sam~ process that resulted in the emplacement of the Phalaborwa Complex farther south (Du Toit et al., 1983).

The metamorphic transition between the Southern Margin­ al Zone and the Kaapvaal craton has been identified and mapped by Du Toit and Van Reenen (1977) in the Hout river -- south of the village of Dendron -- where fairly undisturbed greenstone material exhibits, northwards -- within a distance of 10 km -- progres­ sive structural and mineralogical changes until trans­ formed greenstones occur in the granulite grade of metamorphism. At the same time, the unfoliated granitic material, northwards, turns into foliated gneisses. This transition is further substantiated by petrochemical data.

2.2.2. Granite-greenstone terrane

The Pietersburg and Giyani Groups are two examples of 13 typical greenstone material in the northern portion of the Kaapvaal.craton, and give rise to the Pieters­ burg and Sutherland greenstone belts respectively (figure 2).

According to Prinsloo (1977; cited by S.A.C.S., 1980), the Giyani Group of the Sutherland belt comprises ultramafic ch10rite~amphibolite-talc-serpentine-rich rocks which predominate over mafic amphibolites, acid igneous and sedimentary rocks. Chemical analyses confirm that the ultramafic and mafic rocks were originally peridotitic and basaltic komatiites, meta­ tholeiites and magnesium-rich basalts. Apart from some spinifex textures, virtually no original igneous structures and textures have survived the deformation and metamorphism which varied from upper chlorite to lower amphibolite facies. The associated felsic and sedimentary rocks are volumetrically unimportant. The former are acid intrusions, lavas and tuffs, while the sedimentary rocks comprise phyllite, banded iron-forma­ tion and dolomite.

The Giyani Group is surrounded by grey tonalitic granitoids locally known as the Klein Letaba Gneiss. According to Prinsloo (1977; cited by S.A.C.S., 1980), the greenstones are preserved in a series of synformal structures which are surrounded by large "gregarious" antiforma1 structures in the granitoids. These are not individual intrusives but are domes which probably developed as a result of the synclinal descent of th~ greenstones in an unstable early Archaean crust.

No age determinations have as yet been made for the Giyani Group. It is nevertheless thought to be of the same order of age as the Barberton Sequence, Gravelotte and Pietersburg Groups (S.A.C.S., 1980). "

14

The Pietersburg Group comprises six units (Grob1er, 1972; and S.A.{.S., 1980). Two major cycles of dominantly ultramafic metavo1canics and sialic lavas form the two lower formations. They are conformably overlain by amphibolite originally pillowed mafic meta1ava and mafic metavo1canics. The two upper forma­ t tons.conststs of quartz-carbonate-ch1orite and quartz­ i sericite schist, unconformably overlain by clastic .I I sediments. !

The different type~ of granitic rocks surrounding the various formations of the Pietersburg Group show in­ trusive relationship. The most widespread type is a 1eucocratic biotite gneis$ whose foliation is conform­ able to th~ lineations and major trends of the Group. This 1eucocratic gneiss is thought to have acted as a basement to the Pietersburg Group (Grob1er, 1972; cited by S.A.C.S., 1980).

Other granite plutons, termed Turf1oop, Lunsk1ip and Uit100p (S.A.C.S., 1980), are intrusive into both the 1euco-gneiss and the Pietersburg Group. These granite plutons are homogeneous, coarse-grained to porphyritic and contain biotite and minor hornblende; their radio­ metric ages range between 2330 and 2630 Ma (Burger and Coertze, 1975-1976), which shows that the Pietersburg Group is older than about 2600 Ma.

2.3. Proterozoic to Phanerozoic cover sequences

Large areas of the Limpopo Belt and the Kaapvaa1 craton are obscured by Proterozoic to Phanerozoic cover sequences (figure 2). In the study area, they include 15 the Waterberg Group, Soutpansberg Group and Karoo Sequence, as well as Kalahari, Cretaceous, Pleistocene and Recent deposits.

2.3.1. Waterberg Group

The Waterberg Group consists of a predominant arena­ ceous succession -- mainly coarse-grained sandstones with subordinate amounts of mudstone, siltstone and shale, and minor intercalations of conglomerate and amygdaloida1 lava (in the basal unit)(S.A.C.S., 1980).

The Waterberg basin is one of the several Proterozoic basins that developed on the Kaapvaal craton. Its initial stage in the central Tra~svaal is represented by the Ny1stroom protobasin in which the lowermost beds were laid down. The Nylstroom protobasin sub­ sequently evolved into the Alma trough, whereas the evolution of the late Waterberg basin was largely determined by events in the Limpopo Belt, i.e. meta­ morphic events followed by uplifts and erosion (Jansen, 1976; Coertze et al., 1977).

In the southeastern part of the Waterberg basin, it is thought that no major time interval elapsed between the intrusion of the Bushveld Complex and the deposi­ tion of the Waterberg Group. The lower age limit for the Waterberg Group is about 1920 Ma (Coertze et al .~ 1978) and the middle part is dated at about 1700 Ma (Jansen, 1976), whereas the upper age limit of the Group has not yet been determined. 16

2.3.2. Soutpansberg Group

The Soutpansberg Group comprises a predominantly volcanic lower portion with locally developed sediment­ ary rocks at the base, a predominantly arenaceous mid­ dle portion with interbedded lava flows, and an arenaceous and argillaceous upper portion with inter­ bedded lava flows. The lavas are predominantly of basaltic composition, and the sediments vary in compo­ sition from greywackes to orthoquartzites (Jansen, 1975; Barker, 1979).

The Soutpansberg succession was laid down in an elongated, fault-bounded depression which developed along a major zone of weakness between the Central and Southern Marginal Zones of the Limpopo Belt (refer 3.3.1.).

Radiometric data derived from basal lava indicate a lower age of 1770 Ma (Barton, 1979). The Soutpansberg Group is unconformably overlain by the Karoo Sequence of Permian to Jurassic age (from about 250 Ma to about 140 Ma).

2.3.3. Karoo Sequence

In the northern Transvaal, the Karoo Sequence -- bot~ in the Lebombo and Soutpansberg regions -- comprises ten formally recognized units of which the lower eight ones are composed of sedimentary rocks, including diamictite at the base. The rest of the sedimentary portion of the succession comprises sandstone, carbonaceous shale, coal seams, mudstone and rare 17 conglomerate. The upper two formations mainly com­ prise basaltic lava and rhyolite (Brandl, 1981).

2.3.4. Cretaceous to Recent

Post-Karoo sediments partly cover the older units, e.g~ sandstone and conglomerate of the Malvernia Forma­ tion (Cretaceous) in the eastern part of the study area, and Tertiary to Quaternary deposits -- alluvium, sand, scree, gravel, calcrete -- throughout the study area. 18

3. Structure 3.1. Introduction

To facilitate the description, the study area has been subdiyided into seven domains as shown on figure 3. These are: Bande1ierkop (1), A11days (2), Southern Beit Bridge Complex (3) and Western Transvaal (4), in the Archaean basement; and Soutpansberg (5), Waterberg (6) and Lebombo (7), in the Proterozoic to Phanerozoic cover sequences.-

The structural map of the northern Transvaal, which is presented on Map 1 to be found in the folder inside the back cover, has been provided with a grid system to help with the identification of the features men­ tioned on the following pages. The squares are label­ led A to F and 1 to 13, and wi11 be referred to as such in the text.

LOCALITY

o,

Figure 3. Tectonic subdivision of the northern Transvaal. (1: Bandelierkop domain, 2: A1ldays domain, 3: SoutherQ Beit Bridge Complex domain, 4: Western Transvaal domain, 5: Soutpansberg domain, 6: Waterberg domain, and 7: Lebombo domain) 19

3.2.00mains of the Archaean basement 3.2.1. Bandeliarkop domain 3.2.1.1. General statement

The structural form lines of the Bandelierkop domain depicted on Map 1 are an interpretation based on two unpublished maps of the South African Geological Sur­ vey (2328 Pietersburg sheet - provisional, 1:250 000; and 2330 Tzaneen sheet - provisional, 1:250 000) and reconnaissance maps by Ou Toit, 1979 (Levubu sheet, 1:100 000; and Bandelierkop sheet, 1:100 000, exclud­ ingthe area lying immediately north of the Matok pluton, square C9 on Map 1, which is based on a recent map by Smit, pers. comm.) and Prinsloo, 1977 (Giyani sheet, 1:100 000).

3.2.1.2. General description

The Bande1ierkop domain (1 in figure 3) is subdivided into area 1 (confined to the Southern Marginal Zone of the Limpopo Belt; squares C8, C9 and northern por­ tion of C10, on Map 1) and area 2 (corresponding to the northern portion of the Kaapvaal craton dealt with in this study; squares 08, 09,010,011,012, Cll and southern portion of Cl0, on Map 1).

3.2.1.2.1. Area 1

The most conspicuous structural feature of the area 1 of the Bandelierkop domain (Bd1) is the highly disrupted nature of the xenoliths of pelitic and mafic supra- 20 crustals, distributed in a "seall of granitic material. The supracrustal rocks, termed Bandelierkop Formation, are considered to represent fragmented synclinal ke~ls surrounded by the Baviaanskloof Gneiss (Du Toit and Van Reenen, 1977).

Despite the poor outcrop of rocks in Bdl, it is pos­ sible to distinguish several structural trends. The most remarkable is an approximately west-northwest fold trend which reflects the orientation of the axial traces of a younger folding event superimposed upon an older folding event. The older folding event is revealed by the presence of hook-shaped folds which are not equally obvious over the entire Bdl. The area immediately west of the Ntabalala shear zone, from the Soutpansberg to the northern vicinity of the Matok pluton, exhibits a fair number of well-defined hook folds (see C9, immediately west of the Ntabalala shear zone).

The area where the hook folds are better displayed coincides with the UZwartrandjes-Baviaanskloof, Louis Trichardt and Bochkopjes-Mahilaskop domainsll~ as termed by Du Toit (1979), who considers this earlier folding to be approximately east-west oriented, consisting of an earlier IIFl u and a coaxial IF2" period of folding.

The area immediately north-northwest of the Matok pluton, where the west-northwesterly fold trend is most conspicuous, coincides with the uSand River do­ main" of Du Toit(1979). In this latter area, the. rocks of the Bandelierkop Formation and the granitic phase of the Matok pluton display strong west-north­ westerly, steeply-dipping fabric (Du Toit, 1979). 21

The stereographic projections of the lineations and foliation: plane-s of the "Sand River domain" also sug­ gest the superimposed nature of the west-northwest ori ented f oldi ng eve nt, termed "F3 II by 0u Toit, 1979 (see figure 4). This is clear from the distribution of the lineation poles along the axial plane of the IIF3 11 fold.

TN TN

Figure 4. Schmidt equal area stereographic projections of: (a) 470 foliation planes and (b) 95 1ineations, measured in the IISand River domain". (after Du Toit, 1979)

The eastern portion of Bdl (east of the Ntabalala shear zone) displays more complex fold patterns be­ cause a third, northeast oriented fold trend, which is more typical of area 2 of the Bandelierkop domain .. (Bd2)(refer 3.2.1.2.2.), can also be recognized, while the other two fold trends become less prominent. The time-relationship between the west-northwest- and the northeast oriented folds -- as depicted in this portion of the Bandelierkop domain -- is not clear. i 22

3.2.1.2.2. Area 2

In contrast to the highly disrupted remnants of supra­ crustal rocks of the Southern Marginal Zone of the Limpopo Belt (refer Bdl), the overall outcrop pattern of area 2 of the Bandelierkop domain (Bd2)(the northern portion of the Kaapvaal craton) is characterized by much bigger xenoliths of supracrustal rocks, i.e. the greenstone belt relicts.

The greenstone belts referred to were subjected to a lower grade of regional metamorphism than those of Bdl (refer 2.2.1.), and represent a higher crustal level than Bdl.. Most greenstone belts in the Kaapvaa1 craton underwent lower greenschist facies of regional meta­ morphism (Anhaeusser, 1981).

As demonstrated by Van Reenen and Du Toit (1977), there exists a metamorphic transition between the Southern Marginal Zone of the Limpopo Belt and the Kaapvaal craton, hence between areas 1 and 2 of the Bandelierkop domain. This fact accounts for Bdl and Bd2 not having been taken as two independent structural domains in this study.

Bd2 exhibits fold trends varying from northwest to northeast in orientation. In this area there is good evidence for the later nature of the approximately northwest- to west-northwest oriented folding event". which could be responsible for the variation in orientation of the fold trends ranging from east-north­ east to northeast.

Accordingly, two main fold trends are envisaged in Bd2, viz. 1) a northwest to west-northwest (considered to 23 be the same as the west-northwest fold trend of Bdl); and 2) an east-northeast to northeast fold trend.

The oldest fold event ("Fl-F2" of Ou Toit, 1979) al­ though not clearly noticeable, is also present, par­ ticularly in the area between the Ntabalala and Kudus River shear zones (C9 and C10). This confirms that the structural trends of Bdl and Bd2 are not strictly confined to 'either area, but that some overlapping does exist:

Both the east-northeast to northeast-, as well as the' northwest to west-northwest oriented fold trends are clearly displayed ov~r most of Bd2. The following are examples: i. An approximately northeast-, and a later northwest oriented folding event are displayed in the Pieters­ burg greenstone belt (Grobler, 1972)(square 09); ii. An approximately northeast-, and a later northwest folding event are also clearly displayed in the Suther­ land greenstone-granite terrane (Prinsloo, 1977)(Cll); iii. The "mushroom" shape of the Schiel Alkaline Com­ plex points to a typical folding interference pattern caused, in this case, by approximately west-northwest and northeast oriented folding events. This in no way need imply that the Schiel Alkaline Complex itself, which has virtually no foliation nor has been subjected to metamorphism (Ou Toit et al., 1983), has been folded. The Schiel Complex may have been emplaced in an older structure, that is, the "mushroom"-shaped structure could be the result of an older interference pattern of approximately west-northwest and northeast oriented 24

folding events (C10). It is noteworthy that the folia­ tion in the surrounding Baviaanskloof Gneiss tends to be parallel to the contact with the Schiel Complex (Du Toit et a1., 1983); iv. In the area between the Ntaba1a1a and Kudus River shear zones (southeastern corner of C9 and southern half of C10), that is the vicinity of the boundary between the Southern Marginal Zone of the Limpopo Belt and the Kaapvaa1 craton, the two fold trends are not clearly displayed. This fact is due to the extremely poor nature of the outcrops of the Bande1ierkop Forma­ tion. However, the stereographic projections of folia­ tion plane and lineation measurements in this area seem to indicate the existence of superimposed folding. In figure 5 the lineation poles of the east-northeasterly isocl ina1 folding of the 1101 ifantshoek domain" (Du To i t , 1979) are mainly distributed along the axial plane projection of the fold.

a

Figure 5. Schmidt equal area stereographic projection of: (a) 215 foliation planes and (b) 41 lineations, of an area between the Ntaba1a1a and Kudus River shear zones ("Olifantshoek doma in"; Du To i t , 1979) 25 v. The Murchison Range - immediately south of the southern boundary of the study area (09, 010 and 011) displays a distinct east-northeasterly fold trend, also presenting an approximately west-northwesterly cross fold trend (Anhaeusser, 1981).

3.2.1.3. Faults, fractures and shear zones 3.2.1.3.1. General statement

The faults, fractures and snear zones displayed in the Bande1ierkop domain are derived from published and un­ published maps of the South African Geological Survey (2230 Messina sheet, Geological Series, 1 :250 000, 1981; 2330 Tzaneen sheet - provisional,Geo1ogica1 Series, 1:250 000, undated; 2328 Pietersburg sheet - provisional, Geological Series, 1:250 000, undated) and from recon­ naissance maps by Ou Toit, 1979 (Bande1ierkop and Levubu sheets, 1 :100 000) and Prins1oo, 1977 (Giyani sheet, 1:100 000). A reduced version (scale 1:1 000 000) of the map of faults, fractures and shear zones of the Bande1ierkop domain is presented in figure 11.

Strike orientations of faults, fractures and shear zones were determined and classified into 15° inter­ vals beginning from 0°= North.

The length of faults, fractures and shear zones was measured within each class and then the sum of each class was expressed as a percentage of the whole. The results are represented in a rose diagram (figure 6). 26

3.2.1.3.2. Interpretation

The rose di~gram of figure 6 represents the directions of faults, fractures and shear zones for the whole of the Bande1ierkop domai~. The diagram displays a pre­ dominant northeasterly trend concentrating between 30° and 45°, with fewer faults, fractures and shear zones trending east-northeast. The strongly-developed northeast trend is largely determined by the orienta­ tion of a set of shear zones~ i.e. Ntaba1a1a, Ruigte­ v1y and Kudus River shear zones, the latter being the most prominent of all (see C9, C10, C11, D10 and B11; and figure 11).

The northeast trending shear zones referred to above cut the rocks of the Matok pluton, as well as the Ba­ viaanskloof Gneiss and the Bande1ierkop Formation into which the former pluton is intrusive. The Matok pluton, which is transected by the Ntaba1a1a and Ruigtev1y shear zones (C9), is dated at about 2650 Ma (Barton et a1., 1983), hence indicating a maximum age for this set of shear zones. The latter postdate the west-northwest trending folding event (refer Bd1, in 3.2.1.2.1.) to which the basement and supracrustal rocks of the Bande1ierkop domain, as well as the Matok pluton, were subjected (Du Toit et a1., 1983).

On the other hand, the Pa1mietfontein Granite Plutons, dated at about 2456 Ma (Barton et a1., 1983), which"" are unaffected by deformation and emplaced (some of them) in the shear zones (Du Toit et a1., 1983), in­ dicate a minimum age for the latter.

The Kudus River shear zone is the most conspicuous of 27

TN

-+-+-+-f--+----J 90·

Figure 6. Rose diagram of faults, fractures and shear zones ­ Bandelierkop domain. (Total length of faults, frac­ tures and shear zones: 785 km; 0°_15°: 1%, 15°-30°: 3%, 30°-45°: 30%, 45°-60°: 23%, 60°_75°: 17%, 75°• 90°: 10%, 90°-105°: 4%, 105°-120°: 2%, 120°-135°: 5%, 135°-150°: 2%, 150°-165°: 2%, 165°-180°: 1%) the set of northeast trending shear zones within the Bandelierkop domain (it extends from the eastern por­ tion of square 09, to the southern portion of square Bll). It is about 4 km wide and it was followed for about 30 km during reconnaissance mapping by Ou Toit', (1979) and for more than 100 km on LANDSAT imagery in the present study (figure 57). In the north it is overlain by the sediments of the Soutpansberg Group, and in the south (south of the study area) by the Transvaal Sequence. It has a vertical attitude and, like the rest of the set of northeast trending shear 28 zones, displays a left-lateral sense of horizontal movement, which .could account fora possible displace­ ment between the Pietersburg and Sutherland greenstone belts, as suggested by Du Toit (1979).

An age ranging from 2650 Ma to 2456 Ma can be esti­ mated for the northeast trending set of shear zones referred to above (Du Toit et al., 1983).

As seen on figure 6, only a small number of faults and fractures trend northwesterly (7% between 120° and 150°) and even fewer of them trend north-northwesterly (3% between 150° and 180°). Some of the northwest and north-northwest trending drainage and LANDSAT linea­ ments of the Bandelierkop domain ( figures 57 and 58) -- for many of which no correlation with known struc­ tural features has been found -- could, however, originate from fractures and/or faults. Consequently, the northwest and north-northwest trends of faulting and fracturing in this domain may be more common than the present data suggest. The relationship between these lineaments and the northeast trending set of shear zones is not clear.

3.2.1.4. Dykes 3.2.1.4.1. General statement

For both the map of dyke intrusions of the Bandelier~ kop domain (figure 12) and that of faults, fractures and shear zones of the same domain (figure 11), the source of data is the same (refer 3.2.1.3.1.); similar­ ly, the mode of construction of the rose diagram of dyke intrusions of the Bandelierkop domain (figure 7) 29 and that of faults, fractures and shear zones of the same domain (figure 6) is identical (refer 3.2.1.3.1.).

3.2.1.4.2. Interpretation

Throughout the entire Limpopo Belt, and particularly whithin both the Central and Southern Marginal Zones, numerous post-kinematic mafic dykes are exposed and tend to follow the major fracture and fault trends.

In the Bande1ierkop domain a very large number of dykes are exposed (figure 12) and closely follow the dominant trend of faulting and fracturing of this do­ main (figures 6 and ll). The small number of dykes trending west-northwest (figure 7) are almost res­ tricted to the northwestern portion of the Bande1ier­ kop domain, west of the village of Bande1ierkop (figures To and 12).

As seen on figure 12, the northeast trending dykes literally invade the greater part of the domain, some of them extending for distances as long as 40 kilo­ metres. This suggests that significant amounts of crustal extension must have occurred across this direction, and that the dykes have clearly taken ad­ vantage of a pre-existing structural weakness.

Mason (1973) observed that Karoo Sequence and post-. Karoo Sequence igneous activity is rife throughout the Limpopo Belt, with the intrusion of dolerite dykes, sills and sheets, whereas further investigation proved the existence of older events of dyking. In any case, precise information as to the time of dyke emplacement is still lacking. 30

Barton {1979) considers that post-kinematic mafic dykes were intruded into the Limpopo Belt during at least four distinct periods: a) synchronously with the filling of the Transvaal basin, about 2200 Ma ago; b) syn­ chronously with the deposition of the rocks of the Waterberg Group or the intrusion of the granitic phase of the Bushve1d Igneous Complex, about 1900 Ma ago; c) after the deposition of the rocks of the Soutpans­ berg Group, but before the deposition of the rocks of the Karoo Sequence; and d) after the deposition of the rocks of the Karoo Sequence~

TN

\

Figure 7. Rose diagram of dyke orientations - Bande1ierkop domain. (Total length of dykes: 6575 km. 0°_15°: 3%, 15°-30°: 10%, 30°-45°: 25%, 45°-60°: 25%, 60°-75°: 17%, 75°-90°: 7%, 90°-105°: 4%, 105°-120°: 4%, 120°-135°: 2%, 135°• 150°: 1%, 150°-165°: 1%, 165°-180°: 1%) 31

3.2.1.5. Discussion 3.2. 1 .5. 1. Area 1

The area 1 of the Bandelierkop domain (Bdl) comprises the Southern Marginal Zone of the Limpopo Belt which consists of extremely deformed ortho- and paragneisses at granulite and upper amphibolite grades of regional metamorphism. These rocks are considered to represent high-grade equivalents of the granite-greenstone terranes of the Kaapvaal craton, grading into the lat­ ter with no tectonic break (Du Toit and Van Reenen, 1977; Du Toit et a1., 1983; refer 2.2.1.).

Van Reenen (1983) estimated that the pressure to which the rocks of the Southern Marginal Zone of the Limpopo Belt were subjected was about 9 Kb (equivalent to a tectonic cover of at least 30 km) during the first granu1i t e.event (" ~1l II ) , and about 7, 2 Kb (equivalent to a tectonic cover of at least 24 km) that during the second gran u1i t e event (" ~12 "}, This has been i n­ terpreted as an indication that this area was strip­ ped of at least 6 km (1.8 Kb) overburden during the second granulite event, about 2600 Ma ago, contem~o­ raneously with uplift.

On the basis of gravity data, Barton and Key (1981) also envisaged the granulite terrane of the Southern Marginal Zone of the Limpopo Belt to be the result of uplift that occurred along the northernmost portio~·of the Kaapvaal craton. The Bouguer anomaly pattern (Fig.8a) along the northern margin of the Southern Marginal Zone of the Limpopo Belt reveals a broad lineament of high value which has been interpreted to indicate the existence of a thinner than normal, lower density 32

upper ~rust, and the presence nearer to the surface of higher-density rocks which characterize the lower c r us tand upper man t 1e, i.e. the Souther n ~1 argina1

,. .------,..-

\ I~O o km % N

./)~:\.:.<~ PROTEROZOIC AND PHANEROZOIC ROCKS

~, BOUGUER GRAVITY ISOGRAD (10 milligal) ...... ,..... FAULT ZONES BOUNDING THE CENTRAL i ZONE OF THE LIMPOPO MOBILE BELT • ORTHOPYROXENE ISOGRAD

Figure 8 (a) , (see explanation on next page) 33

-30

-co en

~f------

. ! -130"------NMZ cz SMZ

2.65 2.65- 265

2.95 295 2.95 ...... -, _NNW

( b)

Figure 8. (a): Gravity pattern over the exposed portion of the Limpopo Belt; (b): Average gravity profile across the eastern portion of the limpopo Belt, and its interpretation. Assumed average rock densities are shown. NMZ=North­ ern Marginal Zone; CZ=Central Zone; SMZ=Southern Marginal Zone. (After Barton and Key, 1981)

Zone of the limpopo Belt (figure 8a). Accordingly, these workers suggest that a portion of the Kaapvaal craton was thrust over the Central Zone of the limpopo Belt (figure 8b), therefore elevating granulite facies rocks which were eventually to be e~posed by erosion between about 2600 Ma and 1950 Ma ago (Barton and Key, 1981 ). 34

This contention is essentially identical to the geo­ tectonic model for the Limpopo Belt developed by Light (1982), and also coincides with that of option A of the structure proposed by Coward and Fairhead (1980). The interpretation by Coward (1983) of the gravity section across the eastern portion of the Limpopo Belt is also similar to those referred to above. He, however, envisages the southern granulites to have been uplifted above-a regionally-sized ramp.

All these workers agree that-an event of compression must have caused the uplift of the lower crustal rocks, i.e. the granulite facies rocks comprising the Southern Marginal Zone of the Limpopo Belt. However, there is neither consensus as to the mechanism of up­ 1i ft nor a full understanding of it.

It is here suggested that the structure of the South­ ern Margin~l Zone of the Limpopo Belt at about 2600 Ma could be pictured as shown in figure 9(a}, i.e. As a result of compression, lower crustal rocks of the Kaapvaal craton were upthrust from south to north along one or more south-dipping listric thrust fault planes possibly soling into a gently-dipping to flat­ lying basal shear zone.

Since this model reveals the existence of the Kaapvaal craton partly on edge, the progressively higher grade of regional metamorphism (from greenschist to granulite facies) encountered in the Bandelierkop domain, from"" south (Kaapvaal craton) to north (Southern Marginal Zone of the Limpopo Belt), could express the transition from upper crust to lower crust, as exposed on the present surface. ------~"'----'."-­ ~::~-:~NE SOUTHERN MARGINAL ZONE KAAPVAAL CRATON ! LIMPOPO BELT LIMPOPO BELT. granulite facies < greenschist facies reworked greenstone metamorphic greenstone belt

~~~_\be~\t ;~~:2.~~ .y' v : V. 1/' "iso~rads revmna~ts_._ _ S N ___ -, - V"- ~- \_~,~" . "',., . . .. ,,~ ~ '-,,\', '- '.' ". "'-'- .~~~,,,.,~ '.:~:_-'~: <, -, '-. -- .- (a) ,... --~-. "'--.. ----"" -.-. gneissic ~------~=-.-.....-...... '_::::__ fabric -_.~ -_.- ,_. ~/ \-

SOUTPANSBERG FAULT ZONE

N S '~~---~~ v:',v"'v,v \. \..._. v v ~'""~'-'~\ ~~ \ ',~. -, .' <, - 1 .'0 ~~ ',-.".,~" ". '-. --. " " . " - y-- -...... "<, -'" ."~_.,.~.-~----... (b) , . ---_"::.-----.::...~,.~ - . - ' .. - , --...._--, .._-- - ==> w U"I

Figure 9 . Schematic section across the Southern Marginal Zone of the Limpopo Belt (a), and the Soutpansberg fault zone.(b). Not to scale 36

Accordingly, figure 9 (a) depicts the steeply-dipping gneissic fabric -of the rocks underlying the Southern Marginal Zone of the limpopo Belt as having gentler dips downward. An analogous situation is envisaged for the orthopyroxene and orthoamphibole isograds (figure 9 a), the latter marking the boundary between the Southern Marginal Zone of the limpopo Belt and the Kaapvaal craton (Du Toit and Van Reenen, 1977).

This model is not intended to explain how the green­ stones and granites were buried 30 km down, but only discusses the possible mechanism for their later elevation.

3.2.1.5.2. Area 2

As it has been stated before (refer 3.2.1.2.2.), two main trends of folding are envisaged in the area 2 of the Bandelierkop domain (Bd2), viz. a northwest to west-northwest- and an east-northeast to northeast fold trends.

According to Du Toit (1979), the northwest to west­ northwest fold trend has been "dragged northeastward by the Kudus River shear zone", hence the east-northeast to northeast fold trend would be the result of the effect of deformation of the Kudus River shear zone.

The following observations, however, show that the east-northeast to northeast fold trend represents a folding event per se, independent of the deformational effects that the Kudus River shear zone might have caused upon the surrounding rocks. 37 i) According to Du Toit (1979), original gneiss folia­ tion planes cduld be measured within the 4 km-wide shear zone. This indicates that the deformational effect of the Kudus River shear zone is, at least, not so dramatic as to cause folding up to 30 km away from it (e.g. in the "Na s hamb a " and "Nu l f ma " domains of Du Toit, 1979), as suggested by that author. ii) If the east-northeast to northeast fold trend were to be the result of the effect of deformation of the Kudus River shear zone, the northwest to west-northwest fold trend should disappear to the east of the shear zone. However, the northwesterly fold trend is dis­ played in the Sutherland greenstone-granite terrane (Prinsloo, 1977), east of the Kudus River shear zone. iii) Du Toit (1979) pointed out that the deformation effect of the Kudus River shear zone to the east of it, is far less severe than that to the west of it. The following fact, however, suggests that the deformation­ al effect of the Kudus River shear zone was also not so severe to the west: The Sutherland greenstone belt -- immediately to the east of the Kudus River shear zone -- and the Pietersburg greenstone belt -- immediately to the west of the Kudus River shear zone -- are thought to have been part of a single entity, and to have been displaced by the Kudus River shear zone (Prinsloo, 1977; Du Toit, 1979). Accord­ ingly, both greenstone belts present the same fold trends which predate t he « 2500 f1a Kudus River shear -­ zone.

Therefore, it must be concluded that the Kudus River shear zone did not drag the northwest to west-northwest oriented fold trend to the northeast. Hence the east- 38 northeast to northeast oriented folding must be an event independent of the Kudus River shear zone.

Since the rocks of Bd2 that surround the shear zones are undeformed, whereas those within the shear zones are markedly deformed, a further consequence of the above statement is also implied, i.e. the northeasterly set of shear zones -- of which the Kudus River shear zone is the most prominent -- represents the effect caused by the brittle response of the rocks of Bd2 to the events of deformati9n; whereas the ductile response is confined to very narrow zones, e.g. the 4 km-wide Kudus River shear zone. BANDELIERKOPDOMAIN Scale 1: 1COO 000 ...... •...... ~ .. •• AREA 1 ...-...... -.... Ban~lierkop· ••.•• • .Giyani ...... - ...... ,...... - ...... z· •••••••- • Soekmekaar AREA 2

• Tzaneen Pietersburg.

L __~_~_------t-----A£:"""":==------r------24°S

Figure 10. Locality map - Bande1ierkop domain. BANOELIERKOP DOMAIN Scale 1: 1000 000

// -/ / .... /" / ~~---- fi/ / ;r~ ~ ~ .... '...... ". . //. "'- / f ....: ~

AREA 2 ...... ~ ------

L __~--I------~-_-':"'-+-_-L-=------t------''-24°S

Figure 11. r~p of faults, fractures and shear zones - Bandelierkop domain. (Source of data: refer 3.2.1.3.1.) !-~:::~-----'- ,i BANDELIERKOP DOMAIN

i t I Ill. I

I!

Figure 12. Map of dykes - Bandel·lerkop domain. (Source of data: refer 3.2.1.4.1.) 42

3.2.2. Southern Beit Bridge Complex domain 3.2.2.1. General- statement

The structural form lines depicted in the Southern Beit Bridge Complex domain (see Map l) have been drawn on the basis of the geology of the Beit Bridge Complex as shown on the "Provisional Map of the Limpopo Belt and environs" (Hatkeys, 1983).

3.2.2.2. Main structural features 3.2.2.2.1. General description

The Southern Beit Bridge Complex domain (SBBCd) (3 in figure 3) occupies an east-northeast trending, 25 km average wide belt lying immediately south of the. All­ days domain, and comprises the southernmost part of the Central Zone of the Limpopo Belt within South A­ frican territory.

Tightly folded gneisses and metasediments are exposed in SBBCd (refer 2.2.1.), all strata being strongly aligned in the regionally extensive east-northeast di­ rection. The structural form lines depicted in SBBCd (C5, C6, B9, B10, Ala and All) clearly reflect the orientation of the geological grain in this area.

Extensive faults and shear zones mark the boundaries of SBBCd. For example, in the Messina district, it is bounded in the south by the Tshi pi se fau1 t which in this area (northwestern corner of B10 and southeastern corner of Ala) runs parallel to the east-northeast direction. The Bosbokpoort fault also follows the same direction in the eastern half of SBBCd (B9 and Ala) and, like 43

other faults north of the Soutpansberg region, brings downthrown KarOQ strata into juxtaposition with the Archaean gneisses. Hence it becomes evident that the attitude of the late faulting follows the dominant fabric anisotropy of the Archaean gneisses in this a­ rea (refer (3.3.1.2.).

Also in the Messina district, the SBBCd is bounded in the north by an east-northeast trending linear zone of dislocation showing dextral sense of movement (best seen in figure 19, extending-from the east-central margin of square E6 to the north-central margin of square E8). It is marked over a length of only 12 km in the latter figure, although it might well continue farther east and/or west.

The western portion of SBBCd (C5, C6 and C7) is bound­ ed in the south by a prominent, east-northeasterly trending shear zone known as the Pa1a1a shear zone (McCourt, 1983). This comprises mylonite, ultramylonite and flaser gneiss which crop out in an area of about 10 km wide and 25 km long, whereas Soutpansberg sedi­ ments mask the eastern continuation of the shear zone. McCourt (1983) considers that two events of deforma­ tion took place in the Pa1a1a shear zone, i.e. 1) an older event occurred about 2800 Ma ago, before the main Limpopo deformation (this event is best demonstrated in the "no r t her n subzone", where thin bands of my1o­ nitized felsic material folded at least twi ce are ob­ servable); and 2) a later event occurred about 1850 Ma ago, as it is evident in the "southern s ubzone " which is formed by sheared and my10nitized Pa1a1a Granite dated at about 1856-1893 t·1a (HcCou r t , 1983). The mylonites trend 0700 and dip steeply north (60-70°).

Figure 13 shows a schematic cross-section of the SSBCd 44

traced from about 22°27 IS-30004 IE to about 22°31 'S­ 300l2 1E (see southwestern corner of square A10 for the location of the section line).

1<..00 Sup.'ll'cup (J...... lc)

~ -"'"",'~osillc .111 Supr:f~ol j-.\, rock, _. Slngcl.I.'orItOl.' Of ',hyoli'.' " ',\. 8eilbridge ( ...,1.. - Mat-bl. focies In th. lOV.heost : .. (maInlyquorl.o- , ""1i: • • .. r.ldtpathlc ~.tp.a) ~_~~. __ avor'.l'I.""Offtpillbot'l. onoclollon r : ' ., :. • with iron lamollon ; ,:'1' ( \. !: '-. '~. ;"J : I Basemen' Gneincu with bode dy4t •• """. !-.... :: .... i l: ... C-$ond liver Gn.ln~.·. lorton lLsLI. 197n .. . ~ (~," '. \,"'. .r- .. i ~ .; I ,-1 : , '. ;~ . ' ..., , ..... \>j.~ :-= i : ", ',.\ ., " " '. '.~ ': NW', . \ ~: .. \\ .'

Figure 13. A schematic cross-section of the Southern Beit Bridge Complex domain. (The length of the cross-section is 11 km with no vertical exageration. See southwestern corner of square A10 on Map 1 for the location of the section line). (after Horrocks, 1981)

3.2.2.2.2. Faults and fractures 3.2.2.2.2.1. General statement •

The faults and fractures displayed in the SBBCd (pre­ sented in figure 17) are derived from published and unpublished maps of the South African Geological Sur­ vey (2326 E11isras sheet, Geological Series, 1:250 000, 1959; 2328 Pietersburg sheet - provisional, 45

Geological Series, 1:250 000, undated; 2228 Beit Bridge sheet, Geological Series, 1:250 000, 1957; and 2230 Messina sheet, Geological Series, 1:250 000, 1981), and from the IIProvisiona1 Nap of the Limpopo Belt and e nv t r ons " (Watkeys, 1983).

The mode of construction of the rose diagram of faults and fractures of the SBBCd (figure 14) is identical to that described in 3.2.1.3.1. for the Bande1ierkop do­ main.

3.2.2.2.2.2. Interpretation

Mention has been made in the previous section (3.2.2.2.1.) of the east-northeast fabric orientation of the gneissic rocks comprising the SBBCd, and that the major faults and shear zones extending along the bound­ aries of the domain also follow the same direction. In addition, the main trend of faults and fractures of the entire domain (figure 14) further coincides with the geological grain.

Faults that bring younger Karoo strata (Permian to Jurassic) into juxtaposition with the Archaean gneisses in this region -- notably the Bosbokpoort and Tshipise faults, related to the Soutpansberg fault system -- set the upper age limit Tor the faults of this domain, whereas the lower age limit of the faults and fractu~es cannot be clearly established. 46

TN

270 0 I--t--I-f-...... ,

1800

Figure 14. Rose diagram of faults and fractures - Southern Beit Bridge Complex domain. (Total length of faults and fractures: 534 km; 0°_15°: 1%, 15°-30°: 0%, 30°-45°: 5%, 45°-60°: 3%, 60°-75°: 29%, 75°-90°: 31%, 90°• 105°: 18%, 105°-120°: 13%, 120°-180°: 0%)

3.2.2.2.3. Dykes 3.2.2.2.3.1. General statement

For both the maps of faults-fractures and dyke in­ trusions of the SBBCd (figures 17 and 18 respectively) the source of data is the s ame (refer 3.2.2.2.2.1.);' similarly, the mode of construction of the rose dia­ gram of dyke intrusions of the SBBCd and that of faults and fractures of the same domain is identical (refer 3.2.2.2.2.1.). 47

3.2.2.2.3.2. Interpretation

The directions of dyke intrusions of the SBBCd are shown in figure 15. The latter only represents dykes within the eastern portion of the domain (A10, 89 and B10; and figure 18), since no dykes are displayed in the western portion of the domain (refer 3.2.2.2.3.1.). The rose diagram (figure 15) reveals a predominant east-northeast trend, with fewer dykes trending northeast and west-northwest:

TN

Figure 15. Rose diagram of dyke intrusions - Southern Beit Bridge Complex domain. (Total length of dykes: 434 km; 0°_15°: 2%,15°-30°: 8%,30°-45°: 11%,45°-60°: 12%, 60°-75°: 19%,75°-90°: 29%, 90°-105°; 13%, 105°-120°: 4%,120°-135°: 0%, 135°-150°: 1%, 150°-165°: 1%, 165°• 180°: 1%) 48

Therefore, a marked coincidence exists between the dyke and fault-fracture trends in the SBBCd. This is in accord with the geological evidence (S~hnge, 1945; Bahnemann, 1972; Mason, 1973; Barton, 1979) that in the Limpopo Belt the dykes have been intruded into fractures and faults at several stages.

With the exception of the dykes intruded after "the de­ position of the rocks belonging to the Karoo Sequence", and those intruded before t he deposition of the latter Sequence but after the deposition of the rocks of the Soutpansberg Group, there exists very little direct geological evidence as to the ages of the dykes.

In addition to the two groups of dykes referred to above, Barton (1979) recognized the existence of two further episodes of dyke emplacement on the basis of Rb-Sr isotopic dating. These are dated at about 2200 Ma arrd 1900 Ma.

Barton (1979) also emphasized the fact that unmeta­ morpho sed mafic dykes were being emplaced with chilled margins about 2200 Ma ago, indicating that the major periods of regional thermal metamorphism associated with the Limpopo Belt were finished by then, whereas that any more recent igneous activity must have been local in extent.

3.2.2.3. Interpretation of the main structural features 3.2.2.3.1. Earlier workers

The SBBCd includes part of Bahnemann's (1972) "Linear Be1t" for which he postulated that an overall shearing 49 stress of east-northeast orientation would explain the fold structure within that Belt. He described: (1) folds of similar type with vertical or steeply-dipping axial planes whose traces strike 070°, and axes pos­ sibly plunging to the southwest (no magnitude is given), and (2) highly attenuated folds within the folds of similar type described in (1) above, which would in­ dicate a state of still higher mobility, and that would have taken place during granulite facies of metamorphism. Furthermore, he made use of the concept of a strain ellipsoid to postulate the existence of two conjugate shear directions to produce the folds, and mentioned that at the stage when the folds of the type describ- ed in (2) above were formed II ... the angle between the two directions of shear folding must have approached to zero, therefore the fold trend (described in 2 above) must have been almost parallel to the regional s he ar ".

The mechanism of deformation postulated by Bahnemann (1972) would seem to apply to a process of relatively brittle deformation because of the presence of the two proposed shear directions. Instead, in a ductile shear zone, flattening and extension of viscous rock material are the expected processes.

In an area south of Messina (southeast corner of A9 and southwest corner of A10) which is not confined to either the lILinear Belt ll or the IICross-folded Belt ll of Bahne­ mann (1972) but includes small portions of both, Horrocks (1981) found that the predominant fold axis plunges about 55° to the south-southwest, whereas other, less frequent fold axes plunge 60° and 80°, also to the south-southwest (refer 3.2.2.3.2.). 50

3.2.2.3.2. Discussion

A marked contrast in structural style exists between the SBBCd and the· surrounding co~ntry, and it is eVidently the most important feature to be considered in an attempt to fully understand the structural pat­ tern of this area.

This contrasting difference is plain as the SBSCd represents a narrow strip-like tectonic unit that conveys a relatively higher strain than the surround­ ing Precambrian terranes. This clearly indicates the presence of heterogeneous strain which must have re­ sulted from the development of a shear zone. The type of shear movement (along this shear zone) is the next question that needs to be answered.

Wrench type of movement would signify a shear movement of east-northeast orientation in the shear zone.

If thrusting were the expected mechanism, the present land surface, i.e. the SBBCd, could be envisaged as a natural cross-section developed at a steep angle to the shear movement. Hence the fold axes measured on this cross-section should plunge at equally steep an­ gles, as the former are parallel to the shear move­ ment.

In view of the steeply-dipping, south-southwest oriented fold axes obtained by Horrocks (1981), it would appear that -- at least in the area studied by him; refer 3.2.2.3.1. -- a possible thrust movement would have been predominantly towards the north-northeast. 51

The elliptical outcrop patterns often encountered in . - the SBBCd could represent sections of sheath folds and further reinforce the thrust model referred to above. 27°E 2t"~------_1-_------~---!_------_--L._------_..a.-_---- -, ALLDAYSl and .SOUTHERN BElT BRIDGE COMPLEX 2 DOMAINS

SCALE 1: 1 000 000 1

• ALLDAYS

•

·2

Figure 16. Loca~ity map - Alldays and Southern Beit Bridge Complex domalns. SOUTHERN BElT BRIDGE COMPLEX 2 DOMAINS

~----~ ..,'-.. '\... SCALE 1: 1 000000

". .

...... -:::. ---- ...... " -... ' ...

Figure 18. Map of dykes -A11days and Southern Beit Bridge Complex domatns , (Source of data: 'refer 3.2.2.2.3.1. and 3.2.3.2.3.1. ) 27°'E 22° ALLDAYS1 and SOUTHERN BElT BRIDGE COMPLEX 2 DOMAINS

~----~ ~ .. •• '\..- SCALE 1: 1 000 000 ,-

...... ~ -- --- " .. - ~ ...... -e-..... ­ -... ' ..

Figure 18. Map of dykes -Alldays and Southern Beit Bridge Complex doma ins , (Source of data: -refer 3.2.2.2.3.1. and 3.2.3.2.3.1.) 55

3.2.3. Alldays domain 3.2.3.1. General statement

The st ru ct ur-a l form 1ines depicted in the Alldays do­ main (see Map 1) have been drawn on the basis of the Precambrian geology of this region, as shown in the IIprovisional Nap of the Limpopo Belt and environs ll (Watkeys,1983).

The space left blank in the proximity of the Limpopo river (A8 and A9) is covered by Karoo sediments that belong to the Tuli trough, outside the study area.

3.2.3.2. Main structural features 3.2.3.2.1. General description

The overall outcrop pattern of the rocks underlying the Alldays domain (2, in figure 3) is that of a com­ plexly folded terrane. A great variety of ortho- and paragneisses are exposed in this domain, including the Sand River Gneisses regarded as basement to the more abundant supracrustal gneisses of the Beit Bridge Com­ plex. Infolded and metamorphosed with these gneisses are ultra-mafic gneisses, termed the Messina Layered Intrusion (refer 2.2.1.).

A first inspection of the portion of the structural map of the northern Transvaal corresponding to the Alldays domain (A8, A9, B6, B7, B8 and B9) permits the recognition of remarkably large-sized folds, the most conspicuous of which trend northwest and north­ northwest; other, far less noticeable, folds trend 56 northeast and, in fact, transitions between these two trends are also apparent (AI, 87, A8 and 88). Scale is a key factor, and this description refers to fea­ tures observable in the 1:500 000 map (Map 1), unless otherwise stated.

The northwesterly structural trend (see e.g. 86, 87, B8 and A9) represents large regional folds, some of them characterized by having tight closures, yet opening southward into very broad folds ending as gentle warps (see southeastern portion of A9 and south­ western portion of A10; and squares D4, E4, F3, F4 and F5, in figure 19). These folds have amplitudes ranging 15 to 25 km, being joined to one another by broad (15 to 30 km wide) regional arcs generally with the shape of the capital letter 1JU" with the opening towards the northwest.

Attention should be directed to the relationship existing between the shape of the regional folds re­ ferred to above and their plunge, since a gentle plunge can enormously accentuate their size. This in fact would seem to be the case. For example, the large fold structure west of the village of A1ldays (it straddles the margin between 87 and 88) has a southerly plunge ranging from 19 0 to 30 0 (Van Reenen, pers.comm.). If this plunge were to be regionally uniform, it would signify that the fold shape is like­ wise exagerated in the entire Alldays domain.

The structural form lines in the area between the open limbs of the regional folds indicate the presence of smaller-scale older folds which are deformed about the large folds referred to above (e.g. large structure extending from 86 in the northwest, to 87 in the south­ east). PHOTOGEOLOGICAl H ...... , (...I, o D E '""- ':125 . .. lEGENt' . I-I __oss...... Y&IlIOIa +------1 ~ n.1S OP .."...... I~~I ~IC ....'SS. I

."HJm"1TI & UU"1lA&U1C I-I aoc••. D .',,"UU G....ITJ ...... ~ _A' CIlMITI .1OI1SO. ~~~~~m~~gJi;~~~~~~~~~-z.-D ~-... ~ -. ~......

.•

~;;'J,;~~: - ~"

Figure 19. Photogeological interpretation of the Messina district. (after Bahnemann, 1972).(reduced to 1:250 000) 58

Folds of northeast orientation are not clearly dis~ played in the regional structural map (Map 1), but are instea~ better defined in Bahnemann's (1972) photogeo­ logical interpretation of the Messina district (scale 1:125 000), a reduction of which is presented in fig­ ure 19 (see e.g. northern portion of square 07 and southeastern corner of square 06, in figure 19). Fig­ ure 19 is included in squares A9 and A10 of Map 1.

3.2.3.2.2. Faults and fractures 3.2.3.2.2.1. General statement

The faults and fractures displayed in the Alldays do­ main (presented in figure 17) are derived from publish­ ed maps of the South African Geological Survey (2230 Messina sheet, Geological Series, 1:250 000, 1981; and 2228 Beit Bridge sheet, Geological Series, 1 :250 000, 1957) and the "Provisional Map of the limpopo Belt and environs" (Watkeys, 1983).

The mode of construction of the rose diagram of faults and fractures (figure 20) is identical to that describ­ ed in 3.2.1.3.1. for the Bandelierkop domain.

3.2.3.2.2.2. Interpretation

Figure 20 represents the directions of faults and frac­ tures of the A11days domain. The rose diagram displays a predominant east-northeasterly trend, concentrating

0 between 75° and 90 , with fewer faults and fractures trending west-northwesterly. 59

TN

1800

Figure 20. Rose diagram of faults and fractures - Alldays domain. (Total length of faults and fractures: 198 km; 0°_15°: O%~ 15°-30°: 1%, 30°-45°: 7%, 45°-60°: 6%, 60°-75°:13%, 75°-90°: 45%, 90°-105°: 23%, 105°-120°: 4%, 120°-180°: 1%)

The strongly-developed east-northeasterly trend of faulting-fracturing largely reflects the occurrence of several regional faults of this orientation wit~in the Alldays domain, e.g. the Messina and the Dowe­ Tokwe faults.

The Dowe-Tokwe fault system (from A8 in the west to A10 in the east) is a major strike-slip fault that extends for over 100 km in an east-northeasterly direction and displays dextral sense of movement (Sohnge, 1945 and 1948; Bahnemann, 1972). 60

The Messina fault (central-western portion of A10) branches out from the Dowe-Tokwe fault following an approximately 060 0 direction up to the Limpopo river, and is also a strike-slip fault with dextral sense of movement (Sohnge, 1945). The relative age of the fault is estimated in terms of the copper deposits aligned along it. The Messina fault is thought (Sohnge, 1945 and 1948) to have partly controlled the location of the copper deposits which have been tentatively dated at about 1000 Ma (Ryan et a l , , 1983) with further re­ mobilizations during Karoo iimes. Ryan et al. (1983) also observed that some of the are bodies have been offset by movements along the Messina fault which has, therefore, had a long history, from approximately 1000 Ma ago to Karoo times (Permian-Jurassic).

The Messina and Dowe-Tokwe faults, as well as other faults in the Messina district (eastern portion of A9), are considered to be contemporaneous (Sohnge, 1945).

3.2.3.2.3. Dykes 3.2.3.2.3.1. General statement

For both the maps of faults-fractures and dyke in­ trusions of the Alldays domain (figures 17 and 18 res­ pectively) the source of data is the same (refer 3.2.3.2.2.1.); similarly, the mode of construction of the rose diagram of dyke intrusions of the Alldays domain (figure 21) and that of faults and fractures of the same domain (figure 20) is identical (refer 3.2.3.2.2.1.). 61

3.2.3.2.3.2. Interpretation

A large Dumber of post-kinematic dykes are intruded into the rocks of the Central Zone of the Limpopo Belt of which the Al1days domain forms part (figure 18).

The rose diagram of dyke orientations of the Al1days domain (figure 21) displays a dominant east-northeast

TN

Figure 21. Rose diagram of dykes - A11days domain. (Total length of dykes: 601 km; 0°_30°: 1%~ 30°-45°: 3%, 45°-60°: 14%~ 60°-75°: 28%, 75°-90°: 30%, 90°-105°: 13%, 105°• 120°: 6%, 120°-135°: 5%, 135°-180°: 1%)

trend concentrating between 75° and 90°, with fewer dykes trending west-northwesterly, therefore closely 62

following the orientation of faults and fractures of the same domain {figure 20). In fact, the dykes have been intruded into fractures and faults, as previously noted by Stihnge (1945) and Mason (1973).

Mention has been made (refer 3.2.1.4.2. and 3.2.2.2.3.2.) of the ages of the dykes intruded into the rocks of the Limpopo Belt, determined either from isotopic dating or direct geological observation. In brief, those ages are: 1) about 2200 Ma, 2) about 1900 Ma, 3) post-Sout­ pansberg Group -- pre-Karoo Sequence, and 4) post-Karoo Sequence.

Therefore both the events of dyke intrusion in the A11­ days domain -- hence also in the Limpopo Belt -- and the development of faulting in the Soutpansberg region are partly contemporaneous and represent different mani­ festations of ~he crustal extension in the Limpopo Bel t reg i an( refer 3. 3 . 1 . 4 . ) .

3.2.3.3. Interpretation of main structural features 3.2.3.3.1. Earlier workers

The study of the Messina district (figure 19) conducted by Bahnemann (1972) includes a small portion of the All­

ll days domain (the IICross-folded Be1t ) and of the South­

ll ern Beit Bridge Complex domain (the IILinear Belt ) which was referred to in the previous chapter (3.2.2.).

He considered that an east-northeast trending regional shear strain -- just as postulated for the IILinear Belt ll would account for the overall fold structure of the

"Cr-o s s c f ol ded Be l t " (see figure 22a). Similarly, he 63

(a)

N

( b )

,>e

Figure 22. (a): east-northeast shear couple proposed by Bahnemann (1972) to explain the fold structure of the "Cross­ folded Belt" (part of the Alldays domain); (b): structural pattern that can result from shear deformation, combined schematically with strain e­ llipse (after Harding, 1974). 64

made use of the concept of a strain ellipsoid to produce two conjugate shear directions which would coincide, according to him, with the axial planes of two conjugate fold directions, i.e. a northeast fold trend (with associated dextral shearing movement) and a northwest fold trend (with associated sinistral shearing movement).

What is highly confusing in the explanation of Bahne­ mann (1972) is that the folds that should be asso­ ciated with the simple shear- produced by a (east-north­ east dextral) shear couple will form along the long axis of the ellipse (see figure 22b), therefore his proposed conjugate folds fail to be consistent with theory. Furthermore, the proposed conjugate shear zones may only be kink bands or, in Coward (1984)'s terminology, "shear bands", that is, second order shear zones.

A smal1-sc~le study of the geology and tectonic setting of the Messina Layered Intrusion (southwestern corner of A10; and figure 2, in Barton et al . , 1979a) allow- ed the latter authors to recognize four main events of folding in the Intrusion (and spatially related base­ ment and supracrustal gneisses, i.e. Sand River Gneis­ ses and Beit Bridge Complex respectively). The earliest r eco gni za b1e even t , II D1", i nvol vedthe 1ar ge- sea1e duplication of the Messina Layered Intrusion and the enclosing rocks about flat-lying axial surfaces, hav­ ing a north-south strike direction with fold hinges plunging gently to the west. Thrust and nappe forma­ tion during IID1" event may, in part, have resulted in the apparently djfferent stratigrafic levels at which the Messina Layered Intrusion occurs. Subsequently, the Messina Layered Intrusion together with the 65

infolded units of the surrounding basement and supra­ crustal gneisses, was folded about large northeast trending, more upright axial surfaces with approximate­ ly horizontal fold hinges, giving rise to the broad repetition of these rock units from northwest to south­ east. This deformation is designated /102 11, and together t h the "01/1 folds and thrusts, it generated smaller- wt , scale lIarrow-head/l-type interference patterns. A third deformation, 1103 11, followed, manifested by re­ folding that was approximately coplanar with 1102 11 fold­ ing but had/non-parallel ford hinges. This resulted in a tightening of the existing structures and caused the boudinaging and necking of some of the infolded supracrustal gneisses. Finally, all gneiss units in the area were folded about approximately east-west trending axial surfaces (1I04 11 event).

3.2.3.3.2. Discussion

Whereas it is realized that a close relationship must exist between the roughly north trending fold struc­ ture of the Alldays domain and the east-northeasterly fold structure of the SBBCd, its precise nature is not c1 ea r .

It is nevertheless reasonable to envisage the north trending folds that dominate the structural picture of the Alldays domain as a late event of deformation, that is, they may have taken place after the develop­ ment of the strong east-northeast fabric anisotropy associated with the tightly folded gneisses of the SBBCd. This is suggested by the fact that the east­ northeast oriented fold structure of the SBBCd would, 66

in places, appear to be rotated about the northerly trending direc~ion, as seen for example in B9 (Map l), and in squares E2, E3, F2, F3, F4 and F5 of figure 19. However, it is also evident that curving hinges of large sheath folds- which implies thrust movement - may as' well give the same configuration, and in this case it would follow that both fold structures referred to above are closely related in time.

It was mentioned (3.2.2.3.) that in an area south of Messina, Horrocks (1981) found that the predominant fold axes plunge at steep angles to the south-southwest. This was determined by measuring minor fold hinge lines and linear structures such as fabric due to alignment of prismatic minerals and grains, as well as rodding and mullion structures, all of which indicate the long axis of the strain ellipse, hence defining the direc­ tion of shear movement (in this case, predominantly to the north~northeast). It seems reasonable therefore to attribute the steeply oriented linear and fold structures determined by Horrocks to a dominantly thrust type movement within the area that he has in­ vestigated.

It is clear that the measurements obtained by Horrocks (1981), if collected regionally, will further define the direction of shear movement in the entire Alldays as well as SBBC domains. 67

3.2.4. Western Transvaal domain 3.2.4.1. General description

The Western Transvaal domain (4 in figure 3, and squares E3, E4, F2, F3 and F4, on Map 1) represents a dome of granitic basement correlated with the Archaean Complex. It occupies an area of about 4000 km 2 partly covered by Tertiary and Quaternary sediments, on which only minimal geological information is avail- a b1e.

The oldest formations -- i.e. the Swaziland System and the metamorphic basic and ultrabasic rocks -- form poorly-exposed scattered remnants in the granitic base­ ment; the latter is intruded by the Modipe Gabbro and the Gaborone Granite. Small outcrops of Ventersdorp lavas and sediments are also exposed (Jansen, 1974).

3.2.4.2. Fractures and faults 3.2.4.2.1. General statement

The fractures and faults displayed in the Western Transvaal domain (figure 27) are derived from a map published by the South African Geological Survey (2426 Thabazimbi sheet, Geological Series, 1:250 000, 1974).

The mode of construction of the rose diagram of frac­ tures and faults (figure 23) is identical to that described in 3.2.1.3.1. for the Bandelierkop domain. 68

3.2.4.2.2. Interpretation

Figure 23 represents the directions of fractures and faults of the Western Transvaal domain; the rose diagram displays a predominant east~northeast fault trend concentrating between 060° and 075°. For the construction of this rose diagram, two possible major fractures or faults inferred from the drainage pat­ tern of the Hestern Transvaal domain and indicated as (1) and (2) on figure 27, have not been taken into account. If the latter possible fractures or faults are also considered, the corresponding rose diagram is that of figure 24, in which two further fracture­ fault trends of north-northeast (0°-015°) and north­ west (135°-150°) orientations become apparent. Notably,

TN

1800

Figure 23. Rose diagram of fractures and faults - Western Trans­ vaal domain. (Total length of fractures and faults: 212 km; 0°_15°: 2%, 15°-30°: 8%, 30°_45°: 7%, 45°-60°: 20%, 60°-75°: 26%, 75°-90°: 18%, 90°-105°: 2%, 105°• 120°: 8%, 120°-135°: 2%, 135°-165°: 0%, 165°-180°: 7%) 69

most dyke intrusions in this domain are orientated in a northwest dir~ction largely concentrating between 135° and 150° (see figures 25 and 28).

The relative ages of the fractures and faults can be inferred from the geological map (refer 3.2.4.2.1.), i.e. the fault indicated with number 3 on figure 27 brings Archaean rocks into juxtaposition with Venters~ dorp lavas and basal Transvaal Sequence; the possible fracture or fault indicated with number 1 on figure 27 cuts across the Archaean ter~ane and, to the north of the domain, continues into Waterberg sediments; the possible fracture or fault indicated with number 2 on figure 27 transects the Archaean terrane and, to the southeast of the domain, continues into rocks belong- ing to the Ventersdorp Supergroup and Transvaal Sequence.

TN

Figure 24. Rose diagram of faults - Western Transvaal domain. (Includes possible fractures or faults indicated as 1 and 2 in figure 27; see text) (Total length of frac­ tures and faults: 342 km; 0°_15°: 23%, 15°-30°: 5%, 30°_45°: 5%, 45°-60°: 12%, 60°-75°: 16%, 75°-90°:11%, 90°-105°: 1%, 105°-120°: 5%, 120°-135°: 1%, 135°-150°: 17%, 150°-165°: 0%, 165°-180°: 4%) 70

3.2.4.3. Dykes 3.2.4.3.1. General statement

For both the maps of fractures-faults and dyke in­ trusions of the Western Transvaal domain (figures 27 and 28 respectively) the source of data is the same (refer 3.2.4.2.1.); similarly, the mode of construc­ tion of the rose diagram of dyke intrusions of the Western Transvaal domain (figure 25) and that of frac­ tures and faults of the same domain is identical (re­ fer 3.2.4.2.1.).

3.2.4.3r2. Interpretation

Figure 25 represents the directions of dykes of the Western Transvaal domain; the rose diagram displays a predominant northwest dyke trend largely concentrating between 135 0 and 150°.

- , This direction is therefore parallel to that of a possible major fracture or fault (2 in figure 27) in­ ferred from the drainage pattern of this domain (see also squares E3 and E4, on Map 1). 71

TN

Figure 25. Rose diagram of dykes - Western Transvaal domain. (Total length of dykes: 1452 km; 0°-15°: 1%,15°-90°: 1%, 90°-105°: 3%, 105°-120°: 9%, 120°-135°: 24%, 135°• 150°: 52%, 150°-165°: 10%, 165°-180°: 1%) 26°E 27°E I I I I I I 2/P5 .I WESTERN TRANSVAAL , DOMAIN ) ~ ( .1•• ,I.1"••' SCALE 1:1 000 000 .,-'.. :/. 24°305 .I /' ...... --•• """DERDEPOORT .."...... \.

I 1250 5

Figure 26. locality map - Western Transvaal domain

"-J N 26°E 27°E I I WESTERN TRANSVAAL DOMAIN -,

SCALE 1:1000000 .,r" :/1: (1) \(2) .I ~/ /// . - ./ / 1/ ~ ~ ~ ~.,~ .. ,/ ./ ~ ----./ .',...-., ~(3) __- ./ ,---,II :--= \ •. \

._------..Lso 5 Figure 27. Map of fractures and faults - Western Transvaal domain. (Source of data: refer 3.2.4.2.1.). (1) and (2): possible major fractures or faults inferred from . drainage pattern, both also appear as lineaments on ERTS imagery (S.Afr.geol.Surv., ~ 1974); (3): see text. W 26°E 27°E ..I WESTERN TRANSVAAL DOMAIN ~( . ~..A SCALE 1:1000000 .~~~ ~'" \,,~ . / ..(\~ ~\'\ ~\\0 ~ ~ ./~~,\~~\?~,,~/:\'0"'" \ \ .. ~., ~\.~~~~~ ~ ,~~, . ~.o ~"\~~ ~~~ :,,~\\ ,- \~'".. ~\ .o .0 "'''-. \\ \\\. ~" "­ \.. ,'\ <,- \

I I 25°5 Figure 28. Map of dykes - Western Transvaal domain. (Source of data: refer 3.2.4.3.1.) ....., ~ 75

3.3. Domains of the Proterozoic to Phanerozoic cover sequences 3.3.1. Soutpansberg domain­ 3.3.1.1. Introduction

The most conspicuous structural feature of the Soutpans­ berg domain (5 in figure 3~ and squares B7, B8, B9, B10, Bll~ B12 and Al1~ on Map 1) is that a relatively nar­ row and long fault zone separates 'mobile belt' environ­ ment to the north from the 'cratonic' environment to the south. Hence observations made in this domain should attempt to explain these facts.

The Soutpansberg domain extends for about 250 km in an approximately east-northeast direction~ and ranges from 10 to 40 km in width, the maximum being in the east.

3.3.1.2. Faults and fractures 3.3.1.2.1. General statement

The faults and fractures displayed in the Soutpansberg domain (figure 33) are derived from published maps of the South African Geological ~urvey (2230 Messina sheet, Geological Series~ 1:250 000, 1981; and 2228 Beit Bridge sheet, Geological Series, 1:250-000, 1957), and the "Photogeological Map of the Soutpansberg Group"~ 1:250 000 (Barker~ 1979).

The mode of construction of the rose diagram of faults and fractures (figure 29) is identical to that des­ cribed in 3.2.1.3.1. for the Bandelierkop domain. 76

3.3.1.2.2. Interpretation

The major role played by faulting in the structural evolution of the Soutpansberg domain will be dealt with in the following discussion (3.3.l.4.).

An east-northeast trend of faults and fractures stands out in the rose diagram of figure 29. The fair number of faults and fractures trending west-northwest, north­ northwest and north-northeast in the Soutpansberg region (see figure 33) account for the relatively even distribution of fault and fracture directions of the remainder portion .of the rose diagram.

All the major strike faults present along the northern edge of the Soutpansberg fault zone -- with the excep­ tion of the northerly dipping Afton, Tshamavhudzi, Zout­ pan and Kranspoort faults {B8, B9, B10 and Bll} -- are normal~nd dip steeply to the south. The west­ northwest trending Siloam fault {B10 and C10}, al­ though orientated obliquely to all the other strike faults, also dips in a southerly direction towards its downthrown side {Barker, 1979}.

West-northwesterly trending faults and fractures are more frequent in the northwestern portion of the Sout­ pansberg region, west of the Siloam fault (Barker, 1979).

Faulting is considered to have occurred -- at least partly -- contemporaneously with volcanism and sedi­ mentation {refer 3.3.1.4. for full discussion}. How­ ever, growth faulting has not yet been proved in the Soutpansberg region since post-Karoo faulting largely obscures the original structural pattern (Jansen, 1975). 77

Accordingly, no comprehensive data as to the age of faulting in the Soutpansberg region is available at present.

TN

180·

Figure 29. Rose diagram of faults and fractures - Soutpansberg domain. (Total length of faults and fractures: 3056 km; 0°_30°: 12%, 30°-60°: 15%, 60°-90°: 24%, 90°-120°: 18%, 120°-150°: 17%,150°-180°:, 15%)

3.3.1.3. Dykes 3.3.1.3.1. General statement

For both the maps of faults-fractures and dyke intrusions of the Soutpansberg domain (figures 33 and 34 respective­ ly) the source of data is the same (refer 3.3.1.2.1.); similarly, the mode of construction of the rose diagram 78

of dyke intrusions of the Soutpansberg domain (figure 30) and that of faults and fractures of the same domain (figure 29) is identical (refer 3.3.1.2.1.).

3.3.1.3.2. Interpretation

Numerous dolerite dykes are intruded into the Soutpans­ berg and Karoo successions, particularly in the east­ ern portion of the domain (see figure 34). In con­ trast, there is a distinct absence of dykes in the west­ ern portion of the domain (figure 34).

Figure 30 represents the rose diagram of dyke orienta­ tions of the Soutpansberg domain. It reveals the pre­ sence of a predominant group of dykes with directions ranging between 060° and 120°, concentrating between 060° and 090°, i.e. east-northeast.

A secondary north-northwesterly trend is also display­ ed in the rose diagram (figure 30). It probably re­ flects the lebombo tectonic province (refer 3.3.3.), as dykes of these orientation are exclusively found in the easternmost portion of the Soutpansberg domain, next to the lebombo domain (see figure 3).

The oldest dykes in the Soutpansberg region are those of diabasic composition which have been intruded into the rocks of the Soutpansberg Group and generally pre-: date the main period of faulting (Brandl, 1981), whereas the younger, more abundant, dolerite dykes are of Karoo and post-Karoo ages. 79

TN

Figure 30. Rose diagram of dyke intrusions - Soutpansberg domain. (Total length of dykes: 863 km; 0°_15°: 5%, 15°-30°: 2%i 30°-45°: 3%, 45°-60°: 6%, 60°-75°: 26%, 75°-90°: 22%, 90°-105°:14%, 105°-120°: 9%, 120°-135°: 2%, 135°-150°: 2%, 150°-165°: 5%, 165°-180°: 7%)

• 3.3.1.4. Discussion

Reference has been made in 3.2.1.2.1. to the region lying immediately south of the Soutpansberg domain, i.e. area 1 of the Bande1ierkop domain (see figure 3), which comprises high-grade metamorphic rocks. The latter are thought to be part of the Kaapvaa1 craton's lower crustal rocks brought to surface as a resu1 t of compress ton across the limpopo Belt about 2600 Na ago (refer 3.2.1.5.1.). On the basis of gravity data, Barton and Key (1981) have interpreted the uplift of these rocks as a conse­ quence of the thrusting of a portion of the Kaapvaa1 80

craton over the Central Zone of the Limpopo Belt. A possible mechanism here suggested for the uplift of these rocks (refer 3.2.1.5.1.) has been represented in figure 9a, which shows the lower crustal rocks to have been upthrust (from south to north) along south-dipping thrust faults soling into a basal shear zone.

Mention has further been made of the large attenuation of the fold structure in the southernmost part of the Central Zone of the Limpopo Belt (refer 3.2.2.2.1.).

Later tensional conditions across the Limpopo Belt be­ gan to occur about 1950 Ma ago (Barton and Key, 1981), with the development of faults, fractures and dykes predominantly along the east-northeast direction, and with the reactivation of the zone of crustal weakness earlier developed along the Central Zone - Southern Marginal Zone border. The latter border became the location of a major fault zone, the Soutpansberg trough, whose early stage of development is dated at about 1770 Ma (Barton, 1979). The structural evolution of the Soutpansberg region resulted predominantly from crustal extension, with the consequent development of extensive normal faults (Barker, 1979 and 1983).

The Soutpansberg stratigraphy comprises a cyclic se­ quence of lavas and sediments, striking roughly east­ west and dipping gently to the north (refer 2.3.2.). As indicated from detailed mapping by Meinster (Jansen, 1975) in the Sibasa Formation west of Louis Trichardt, faulting was active during the eruption of the lavas. This is consistent with the development of basaltic lavas in regions undergoing extensions (Gilluly, 1963; Barker, 1983). The greater number of the major strike 81

faults present in the Soutpansberg region are normal and dip steep1y"to the south (Barker, 1979).

According to Anderson et a1 .(1983), the geometry of the slip on concave-upward (listric) normal faults re­ quires horizontal extension in excess of that pre­ dicted from the steep near-surface fault dip and re­ sults, geometrically, in a potential void (figure 31 B1). Thus, to accommodate this space problem, major slip (typically 3-6 km) accompanying basin formation on m~ster listric faults should be accompanied by a conspicuous pattern of down-bending or flexing of the basin-fill strata into the fault (figure 31 B1-B4) and/or antithetic faulting (figure 31 A), as well as associated growth-fault pattern of sedimentation (fig­ ure 31 B4).

It could be argued that these three conditions are present in_the Soutpansberg fault zone, i.e. 1) Downbending of basin-fill strata into the faults could account for the northerly dip of the Soutpans­ berg succession; 2) Antithetic faulting could explain the occurrence of infrequent north-dipping faults; and 3) Growth-fault pattern of sedimentation implies con­ temporaneous faulting, and the latter has been encoun­ tered in the Soutpansberg region (Jansen, 1975) by non-reactivated faults that intersect only the lower volcanic portions of the Soutpansberg succession.

Accordingly, it is here suggested that a model featured by south-dipping, normal listric master faults soling into a gently-dipping, major detachment surface could adequately explain the overall structure of the Sout­ pansberg domain (see figure 9b). 82

It is further envisaged that the system developed earlier in the geological history of the region along the Central Zone" Southern Marginal Zone border, and which is characterized by south-dipping thrust faults soling into a basal shear zone (figure gal, could have controlled the late extensional displace­ ment system referred to above.

------:

A

------

Figure 31. Cross-sectional sketches showing some depositional and rotational characteristics of strata that are displaced by listric faults. Normal movement along the curved fault would tend to pull the blocks apart as well as displace them vertically, and subsidence into the gap may develop antithetic faults (A) or rotation of strata (Bl-B4). (after Anderson et al., 1983)

The possible interrelation here suggested to exist bet­ ween early thrust tectonics and late extensional tec- 83

tonics in the region between the Central and Southern Marginal Zones of the Limpopo Belt is similar to that encountered in the eastern Basin and Range province, USA.

Recent work by Allmendinger et al. (1983) based on COCORP seismic-reflection data provides information on the interrelationship between Mesozoic thrusting and Cenozoic extensional tectonics in that area. Those data suggest that the crustal structure of the eastern Basin and Range province is dominated by a series of low-angle detachment surfaces that can be traced over horizontal distances of 70 km or more and depths between 12 and 15 km. Many of these surfaces (reveal­ ed as remarkably continuous, low-angle reflectors in the COCORP seismic sections) have, on a regional scale, a seismic character normally associated with thrusts. However, no high-angle normal faults known in the near surface extend deep into the crust but instead are either truncated or sole into the low-angle detachments, suggesting that the latter are, in their more recent history, low-angle normal faults, some with perhaps many tens of kilometres of Cenozoic extensional dis­ placement. 30°

'SOUTPANSBERG DOMAIN

SCALE 1: 1000 000

Figure 32. Locality map - Soutpansberg domain. 30°

SOUTPANSBERG DOMAIN

SCALE 1:1000 000

----

Figure 33. Map of faults and fractures - Soutpansberg domain. {Source of data: refer"3.3.l.2.l.} {f: fault that coincides with boundary of the domain} "" " 300

SOUTPANSBERG DOMAIN

SCALE 1:1000000

--' / ­ \

230

Figure 34. Map of dykes - Soutpansberg domain. (Source of data: refer 3.3.1.3.1.) 87

3.3.2, Waterberg domain 3.3.2.1. General-statement

The structural form lines depicted in the Waterberg domain (see Map 1) have been drawn on the basis of the geology of the Waterberg Group as shown on the map "Geology of the Waterberg basins in the northwestern Transvaal" (Jansen, 1982), scale 1:250 000, which ac­ companies the Memoir 71 of the South African Geologic­ al Survey.

Two relatively small outcrops of rocks belonging to the Karoo Sequence and Bushveld Igneous Complex are in­ cluded in the northwestern portion of the Waterberg domain (C4, C5, C6, 04, 05 and 06). For the sake of clarity they have not been indicated as subdivisions on Map 1, although due consideration was given to them in the overall structural interpretation.

3.3.2.2. General description

The Waterberg domain (6 in figure 3) is confined to the Waterberg basin. Its main structural features are de­ picted on the regional structural map (C6, C7, 04, D5, 06, D7, E4, E5, E6, E7 and F6), and can be summarized as follow: i) an approximately northeast trending major downwarp cuts across the domain in its central portion (it ex­ tends from the northeastern corner of F4 to the south­ ern portion of C7); 88

i1) less well defined, approximately northeast orient­ ed warps probably related to (i) are visible to the north and parallel to (i) (e.g. northwestern corner of E5 to southeast corner of E3 within the Western Transvaal domain; centre of 04 to northeast corner of E3; northeast portion of C7 to northwest portion of 06 within the Bushveld Igneous Complex); iii) an approximately northwest oriented major down­ warp transects the central portion of the domain (i.e. central-western 'portion of 05 to centre of E6); iv) a less well defined, a ppr-o xima t el y northwest orient­ ed upwarp probably related to (iii) is visible to the north and parallel to (iii) (i.e. southwest portion of C6 to, centre of 07); v) locally intense deformation in the south~ and southeasternmost parts of the Waterberg domain produced folding, faulting and thrusting (southern portion of E6, E7, F4, F5, F6 and F7). The folds are anticlinal, syn­ clinal and monoclinal, and their axes display general lack of parallelism (Jansen, 1982). Faulting also at­ tained its maximum intensity in this area which co­ incides with the oldest portion of the Water berg suc­ cession; vi} few major faults cut across the domain, notably the west-northwest trending Vaalwater fault (south­ western corner of 04 to central-western portion of E6) and the east-northeast trending Melinda fault extending along part of the northern boundary of the domain (southeastern portion of C4 to northwestern portion of C7) • 89

The major northeast and northwest trending downwarps referred to above, i.e. (i) and (iii) respectively, intersect in the central area of the domain (south­ western portion of D6, northeastern portion of E5 and northwestern portion of E6), and it is in this area where the youngest part of the Waterberg succession {the Vaal water Formation} has been preserved.

It would therefore seem that the overall structural picture of the Waterberg domain is controlled by the superimposition of northeast- and northwest oriented gentle fold trends. It would further appear that -- with the exception of the locally intense deforma­ tion in the early Waterberg basin referred to in (v) -­ the greater part of the Waterberg domain developed on a relatively stable portion of crust.

3.3.2.3. Faults and fractures 3.3.2.3.1. General statement

The faults and fractures displ~yed in the Waterberg domain {see figure 38} are derived from published and unpublished maps of the South African Geological Sur­ vey (2328 Pietersburg sheet - provisional, Geological Series, 1 :250 000, undated; 2326 Ellisras sheet, Geo­ logical Series, 1:250 000, 1959; 2426 Thabazimbi sheet, Geologica] Series, 1:259000,1974; 2428 Nylstroom sheet, Geological Series, 1 :250 000, 1978) and from the map, scale 1 :250 000, IIGeology of the Waterberg basins in the northwestern Transvaal II (Jansen, 1982).

The mode of construction of the rose diagram of faults and fractures (figure 35) is identical to that des­ cribed in 3.2.1.3.1. for the Bandelierkop domain. 90

3.3.2.3.2. Interpretation

The rose diagram of faults and fractures of the Water­ berg domain (figure 35) reveals a predominant trend ranging from 060° to 120°, largely concentrating bet­ ween 90° and 120°, i.e. west-northwest.

Faults in this domain are of normal type and post~ Waterberg Group in age (Jansen, 1982), and at least some of them wer~ active in post-Karoo times, e.g. the Helinda fault.

TN

1800

Figure 35. Rose diagram of faults and fractures - Waterberg domain. (Total length of faults and fractures: 478 km; 0°_30°: 5%, 30°-60°: 7%, 60°-90°: 21%, 90°-120°: 54%, 120°-150°: 9%, 150°-180°: 3%) 91

3.3.2.4. Dykes 3.3.2.4.1. General statement

For both the maps of faults-fractures and dykes of the Waterberg domain (figures 38 and 39 respectively) the source of data is the same (refer 3.3.2.3.1.); similarly, the mode of construction of the rose diagram of dyke intrusions of the Waterberg domain (figure 36) and that of faults and fractures of the same domain (figure 35) is identical (refer 3.3.2.3.1.) ..

3.3.2.4.2. Interpretation

Dyke intrusions in the Waterberg domain follow mainly west-northwest and northeast to east-northeast direc­ tions, as revealed in the rose diagram (figure 36). These directions are therefore the same as those dis­ played by the faults and fractures of this domain (figure 35), although with a larger number of dykes of northeast orientation.

In the northwestern portion of the domain, the close coincidence between fault and dyke orientations is evident, particularly among west-northwest trending dykes and faults (figures 38 and 39), whereas the same condition is less clear in the rest of the domain.

The dykes are intruded into the rocks belonging to the Waterberg Group, but more precise dyke ages are not available at present. 92

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1800 .

Figure 36. Rose diagram of dyke intrusions - Waterberg domain. (Total length of dykes: 337 km; 0°_30°: 6%, 30°• 60°: 20%, 60°-90°: 23%, 90°-120°:41%, 120°-150°: 5%, 150C?-1800: 5%)

; l t t· WATERBERG DOMAIN

. scs LE 1: 1 000 000

ELLISRAS

•GLENOVER

-ALMA • THABAZIMBI

• NYLSTROOM

Figure 37. Locality map - Waterberg domain 29°

WATERBERG DOMAI N

SCALE 1:1 000000

:..

Figure 38. Map of faults and fractures - Waterberg domain. (Source of data: refer 3.3.2.3.1.) 29°

WATERBERG DOMAIN·

SCALE 1:1 000 000

-,

/ ;' 1/ /

Figure 39. Map of dyke intrusions - Waterberg domain. (Source of data: refer 3.3.2.4.1.) 96

3.3.3. Lebombo domain 3.3.3.1. General rlescription

The Lebombo domain (7 in figure 3) is confined to the northern portion of the Lebombo Range. It extends southwards from the Limpopo river to 23°30'S~ a dis­ tance of about 130 km~ being 2 to 3 kilometres in width.

It comprises sediments and volcanics belonging to the Karoo Sequence which rest on older basement rocks of the Kaapvaa1 craton and the Limpopo Belt (refer 2.3.3.).

The structure of the Lebombo domain is relatively simple and is characterized by a faulted monoc1ine~ the strata striking meridiona11y and dipping 5° to 15° eastward.

The conspicuous Karoo volcanism in this region is thought to be intimately associated with the rifting and monoc1ina1 flexuring which developed prior to and during the fragmentation of Gondwanaland (Flores, 1970). The fragmentation was initiated by the separation of eastern Gondwanaland (India, Antarctica and Australia) from western Gondwanaland (Africa and South America), with the present-day Lebombo representing the tilted margin at the western flank of the rift system which developed during the continental separation (Bristow, 1982) .

The complex history of volcanism and tectonism in this region covered a period of 65 million years, from mid­ dle Jurassic (200 Ma) to Cretaceous (135 Ma){Bristow, 1980; Allsopp et a1., 1981), volcanism having preceded the main period of rifting. 97

3.3.3.2. Faults and fractures 3.3.3.2.1. GeneraJ statement

" The faults and fractures displayed in the Lebombo ,do­ main (figure 41) are derived from the following maps of the South African Geological Survey: 2230 Messina sheet, Geological Series, 1 :250 000, 1981; and 2330 Tzaneen sheet - provisional, Geological Series, 1:250 000, undated.

3.3.3.2.2. Interpretation

No statistical analysis of fault-fracture orientations has been carried out due to the small number of faults and fractures present in the Lebombo domain (figure 41). They are all concentrated in the northwestern corner of the domain, although some of the lineaments inferred from the drainage pattern and LANDSAT imagery of the Lebombo domain (figures 74 and 75 respectively) could also represent faults or fractures.

The faults and fractures seen in the northwestern cor­ ner of the Lebombo domain (figure 41) reflect an inter­ play between the Soutpansberg and Lebombo structural trends, the former represented by east-northeast and west-northwest faults and fractures and the latter represented by roughly north-northwest oriented faults and fractures.

3.3.3.3. Dykes 3.3.3.3.1. General statement 98

for both the maps of fau1ts~fractures and dykes of the Lebombo domain (fJgures 41 and 42 respectively) the source of data is the same (refer 3.3.3.2.1.),

3.3,3.3.2. Interpretation

As for the faults and fractures, statistical analysis has also not been carried out for the dykes of the Le­ bombo domain since they are relatively insignificant in number. Furthermore, they concentrate in the north­ ern portion of the domain (figure 42).

They are intruded into the rocks belonging to the Karoo Sequence, and display mainly north-northwest and west-northwest orientations. 99

r------L------2205

LEBOMBO DOMAIN 23030'

SCALE 1: 1 000 000

Figure 40. locality map - lebombo domain. 100

.------.L..------_22°S

- .. •

LEBOMBO DOMAIN

SCALE 1:1000000

Figure 41. Map of faults and fractures - Lebornbo domain. (Source of data: refer 3.3.3.2.1.) 101

r------IL------....22°S

- ..

LEBOMBO DOMAIN

SCALE 1:1 000 a00

Figure 42. Map of dykes - Lebombo domain. (Source of data: refer 3.3.3.3.1.) 102

4. Aeromagnetic trends of the study area 4.1. Introduction-

In this chapter, the regional trends of the magnetic anomalies are summarized and related, where possible, to the structural features of the study area as refer" red to in the previous sections.

In accord with the scale of the structural map (1:500 000, Map 1), the following pages deal mainly with regional magnetic anomalies, defined as distortions on the magnetic field due to broad crustal structures, and perceptible over large distances, in contrast to local anomalies due to small-scale structures. The in­ terpretation of the aeromagnetic survey of the study area is only qualitative.

The terms low, intermediate to high, and very high magnetic intensities (see figures 43 a, b and c) are relative and refer to the sharpness of the magnetic gradient associated with the anomalies. The magnetic gradient is in turn reflected on the separation bet­ ween magnetic contours.

Lineaments 'have been taken as relatively narrow and long magnetic anomalies (figure 44a). Frequently, sudden swings in the trends of the magnetic contours occur along distinct directions. Where clearly notice~ able, these directions have also been taken as linea­ ments (figure 44b).

The greater number of the lineaments inferred from the aeromagnetic maps are of the type shown in figure 44a. 103

(a)

_24°00 1 $ ~.x.:.::..:==~~~=....:::::l.D..;~':;;"":~~~~~ 27°45 1E

,.

(b)

Figure 43. {Explanation on next page} 104

Figure 43. Examples of magnetic regimes of low (a), intermediate to high (b), and very high (c) intensities. (c) is seen both as a strip extending from the SW cor­ ner to the NE corner of the figure, and in the SE cor­ ner of the figure.

The magnetic contour maps (scale 1:250 000), edited by the South African Geological Survey, on which the fol­ lowing description is based are listed below: 2227 Maasstroom-Tom Burke-Stei1100pbrug area, 2229 Mes­ sina area-sheet 1, 2230 Messina area-sheet 2, 2326 E11is­ ras-Pietersburg-Pha1aborwa area, 2329 Louis Trichardt­ Bandelierkop area, 2426 Thabazimbi-Derdepoort area and 2428 Potgietersrus area.

The strike orientations of the magnetic lineaments were determined and classified into 30° intervals beginning_ from 0°= north. The length of the magnetic lineaments was measured within each class and then the sum of each class was expressed as a percentage of the whole, these results being represented in rose diagrams (figures 45,47,49,51 and 53). ' 105

(a)

(b)

23°15'5 29°30'E 29°45°E

Figure 44. Examples of lineaments inferred from the aeromagnetic maps. (a): relatively narrow and long magnetic a­ nomalies of sharp gradient in the western portion of the figure (See also a broad strip of very high mag­ netic intensity, from central-southern- to central-east­ ern margins of figure, which coincides with part of the Pietersburg greenstone belt; (b): lineaments ex­ pressed as a swing in the trend of the magnetic con­ tours (dotted lines). 106

4.2. Description 4.2.1. Bandelierk~p domain

A first inspection of the aeromagnetic survey of the Bandel t erkop domain reveals a contour pattern of high magnetic intensity with a strongly-developed northeast to east-northeast fabric (figure 46).

There are also smaller areas of low magnetic intensity, e.g. the area underlain by the Schiel Alkaline Complex, where the strong northeast magnetic trend is interrupt­ ed.

The magnetic contour pattern associated with the Bandelierkop domain is interrupted about 5 km to the east of the boundary between the Bandelierkop and le­ bombo domains (see figure 3), within the latter. This magnetic pattern indicates the easternmost extension of the Archaean terrane in this region .

The Sutherland• greenstone-granite terrane does not show any distinct~orrelation with the magnetic contour pat­ tern, whereas the iron-rich units of the Pietersburg greenstone belt are marked by areas of very high mag­ netic intensity.

The Kudus river shear zone is partly associated with a strip-like area along which a subtle inflection in the trend of the magnetic contours is visible (about 23°15 15 - 30 025 IE).

The southern portion of the western boundary of the do­ main is determined by a broad anomaly of very high mag­ netic intensity which originates from an outcrop of 107

rocks of the Bushveld Igneous Complex. No feature on . .' ~ . . the m~gnetic contour map marks the boundary between the Bandelierkop and Waterberg domains.

The background northeast to east-northeast magnetic fabric of the domain is partially masked in areas tran­ sected by a large number of lineaments. The latter generally represent narrow, positive (and fewer negative) magnetic anomalies of hig~ gradient, most of which orig­ inate from dyke intrusions. Many of the lineaments inferred,from the aeromagnetic survey, however, have not been correlated with any further, known geological or structural features. The main strike orientations of the magnetic lineaments are northeast to east-north­ east-, and a secondary ~est-northwest direction (fig- ure 45. Compare with figure 7 showing orientation of dykes) .

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Figure 45. Strike orientations of the undifferentiated lineaments inferred from the aeromagnetic survey - Bande1ierkop domain. (Total length of lineaments: 2012 km; 0°_30°: 11%, 30°-60°: 34%, 60°-90°: 30%, 90°-120°: 14%, 120°• 150°: 7%, 150°-180°: 4%) BANDELIERKOP DOMAIN Scale 1: 1000 000 -:---- i I

...... - :/n:/:~ ; - '-- - / '" . ,. '" .,'", -- ,.' /"'''-'''''- ---j/~ . I // /~ I/~ ~;- ...... ---- ".. '.' , I - / "....;, ..::;::: • /./ /\./I ...:•.;..-----'-...... --....- ....."- ::.:: /-' /~ :::.. i "1'-, • L __~2+-_....:...-_~L---Ll-J~L----l...L.-.L--t-.i...--L-~-=--_·_------t------I_24°S.'/ .'-......

Figure 46. Map of undifferentiated li~eaments inferred from. the aeromagnetic survey - Bandelierkop domain. (Source of data: refer 4.1.) KEY: lineaments c===J intermediate to high magnetic intensity ------1ineaments (not cl early defined) lr:~~~:::'l low magnetic intensity /'/ ./' approximate boundaries of a broad magnetic anomaly of .-.../ .,.... very high intensity .,- ",- 109

4.2.2. Alldays domain - Southern Beit Bridge Compl~x domain region

The region comprlslng the Alldays and Southern Beit Bridge Complex domains presents a regime of intermediate to high magnetic intensity (figure 48).

The two domains, which form part of the Central Zone of the Limpopo Belt, cannot be distinguished on the basis of magnetic contour pattern, despite the clear difference in structural trends that characterizes them (refer 3.2.2. and 3.2.3.). Note e.g. the total lack of correspondence between the roughly north trend­ ing structure of the Alldays domain and the roughly east-west orientation of the magnetic contour trends of the same domain.

A fair number of lineaments are visible on the mag­ netic contour map of this region, which generally re-. present narrow, positive anomalies of intermediate gradient. Although some of the lineaments may orig­ inate from dyke intrusions, no further correlation with any known geological-structural feature has been found for many of them.

The main strike orientations of the magnetic linea­ ments -- east-northeast to west-northwest -- are dis­ played in figure 47. 110

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Figure 47. Strike orientations of the undifferentiated lineaments inferred from the aeromagnetic survey - Al1days and Southern Beit Bridge Complex domains. (Total length of lineaments: 1136 km; 0°_30°: tz; 30°-60°: 10%, 60°-90°: 43%, 90°-120°: 39%, 120°-150°: 7%, 150°-180°: 1%)

4.2.3. Western Transvaal domain

The greater part of the magnetic contour map of the Western Transvaal domain shows a regime of intermediate to high magnetic intensity (figure 50). A relatively large number of lineaments are also visible on the map, mast of them trending west-northwesterly, with a smaller number trending northwest and east-northeast (figure 49). They both represent narrow, positive 3'~E

.ALL-DAYS' and SOUTH·ERN BElT BRIDGE COMPLEX 2 . - ,.../' ~ ~ ../.. DOMAIN·S /"-'.. .. •.

--~---~..,- ~ .., SCALE 1:' 000 000

~~ r'.• ..r-.• ,J.

Figure 48. Map of undifferentiated lineaments inferred from the aeromagnetic _ survey - Alldays and Southern Beit Bridge Complex domains. (Source of data: refer 4.1.; KEY: see figure 46) 112

magnetic anomalies of high gradient, and many of them originate from dykes,

Jansen (1974) considers that many of the west"north" west trending aerial magnetic anomalies in the granitic basement of the western Transvaal region arise from post-Karoo faults.

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Figure 49. Strike orientations of the undifferentiated lineaments inferred from the aeromagnetic survey - Western Trans­ vaal domain. (Total length of lineaments: 258 km; 0°-60°: 0%, 60°-90°: 13%, 90°-120°: 69%, 120°-150°: 18%, 150°-180°: 0%) 26°E 27°E ..I WESTERN TRANSVAAL DOMAIN

SCALE 1: 1000 000

...' ,e:.. :' .., :::...... '," .... :.::'. ":':'.::~, . .. :~~ .. '.:~. ~;:.:. . - ____ •••__.-.,1"..,A·•••••.. ~ ..---.'~ ." '. :.... I.: .•..,: ...... ~..., . ~.' -..:.::::,:.:.: :. ." ...:.,; . :.: ....;.;.;.;.:.:.. ',':...... ;:; ..;::.' ,'., \. \

...' ------IL25° S

Figure,50. Map of undifferentiated lineaments inferred from aeromagnetic survey - Western --' Transvaal domain. (Source of data: refer 4.1.; KEY: see figure 46) --' w 114

4.2.4 Soutpansberg domain

The magnetic contour map of the Soutpansberg domain exhibits a regime of intermediate to high magnetic intensity (figure 52).

The boundary between the Soutpansberg domain and the neighbouring Bandelierkop and Southern Beit Bridge Complex domains cannot be recognized on the magnetic contour map, since the magnetic trends of the latter domains extend a fair distance into the Soutpansberg domain.

No clear correlation between the various lithologic units underlying the Soutpansberg domain and the pat­ tern of the aeromagnetic contour map of this region has been recognized.

Some of the lineaments inferred from the aeromagnetic survey coincide with dykes, fractures and faults (see figures 33 and 34), whereas the possible correlation between some further lineaments and the regional structural geology is not clear.

Figure 51 represents the strike orientations of the magnetic lineaments of the Soutpansberg domain. It reveals a strongly-developed west-northwest to east­ northeast trend. 115

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1800

Figure 51. Strike orientations of the undifferentiated lineaments inferred from the aeromagnetic survey - Soutpansberg domain. (Total length of lineaments: 604 km; 0°_30°: 0%, 30°-60°: 9%, 60°-90°: 42%, 90°-120°: 47%, 120°• 150°: 2%, 150°-180°: 0%)

4.2.5. Waterberg domain

The Waterberg domain represents an area where the Archaean basement -- of higher magnetic susceptibility is covered under a thick succession of non-magnetic sediments belonging to the Waterberg Group. According­ ly, a regime of low magnetic intensity predominates in the greater part of this domain (figure 54), with the exception of some areas close to basement expo­ sures or to other rocks of higher magnetic suscepti­ bility, viz. i) a small area in the proximity of the 30°

SOUT PAN SBERG DOMAIN 1 i I I ! ~~ :~:::. - ; ' ::::--"..... '" - ~;: . SCALE 1:1000 000 ~ :.:. ·······;.;···7 "::;" . -->

23°

Figure 52. Map of undifferentiated lineaments inferred from the aeromagnetic survey - Soutpansberg domain. (Source of data: refer 4.1.; KEY: see figure 46) 117

granite dome of the Western Transvaal domain (refer figures 3 and 54); ii) the northeasternmost portion of the Waterberg domain, close to the exposed basement rocks of the Central Zone of the Limpopo Belt (refer figures 3 and 54); and iii) the area next to an out­ crop of the Bushveld Igneous Complex which defines the southeastern boundary of the Waterberg domain (C7, Map 1). (iii) indicates the continuation of the Bushveld Complex under this area.

The Waterberg domain is transected by a fair number of , lineaments of west-northwest and east-northeast orientations (figures 53 and 54). They generally represent positive (and fewer negative) magnetic a­ nomalies of moderate to high gradient, most of which originate from dyke intrusions (compare to figure 39).

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270· r--f-t-J rl--t--1 90.

180·

Figure 53. Strike orientations of the undifferentiated lineaments infered from the aeromagnetic survey - Waterberg do­ main. (Total length of lineaments: 882 kID; 0°_30°: 0%, 30°-60°: 12%, 60°-90°: 33%, 90°-120°: 46%, 120°-150°: 6%, 150°-180°: 3%) 29°

WATERBERG DOMAIN ---- SCALE 1: 1 000 000 - .... :;::---

-Figure 54. Map of undifferentiated lineaments inferred from the aeromagnetic survey ­ Waterberg domain. (Source of data: refer 4.1.; KEY: see figure 46) 119

4.2.6. lebombo domain

The magnetic contour map of the lebombo domain ex­ hibits a regime of intermediate to high magnetic . "' intensity which arises from the Karoo volcanic rocks underlying the re~ion (see figure 55).

The chief northwest trend of the magnetic anomalies of the lebombo domain contrasts with the east-north­ east trend of the neighbouring Archaean basement rocks of the Bandelierkop domain. The boundary bet­ ween these two magnetic patterns does not coincide exactly with that of the two domains) but is slightly displaced towards the east -- within the lebombo domain -- indicating the easternmost extension of the Archaean basement in this region.

A small number of lineaments have been inferred from the magnetic contour map (see figure 55») although their origin is not clear. 120

,------1------_22°5

-- ..

/ LEBOMBO DOMAIN

23°30' SCALE 1:1000 000

Figure 55. Map of undifferentiated lineaments inferred from. the aeromagnetic survey - Lebombo domain. (Source of data: refer 4.1.; KEY: see figure 46) 121

4.3. Interpretation

The analysis of the regional trends of the magnetic anomalies presented in the previous section includes the description of_both magnetic regimes and lineaments.

It is apparent that the metamorphic Precambrian terranes of the Bandelierkop, Alldays and Southern Beit Bridge Complex domains, as well as the Western Transvaal domain, are largely associated with mag­ netic regimes of intermediate to high intensity. In addition, the Lebombo domain also presents a regime of intermediate to high magnetic intensity that arises from the Karoo volcanics in this region. The same applies to the Soutpansberg domain, although this domain also shows isolated areas of low magnetic intensity possibly related to the presence, nearer to the surface, of portions of the Soutpansberg succession of reduced thick­ ness of volcanics and/or increased thickness of sedi­ ments.

On the other hand, the greater part of the Waterberg domain, which is underlain by a thick succession of non-magnetic sediments, shows a magnetic regime of low intensity.

Most of the lineaments inferred from the magnetic con­ tour maps correlate with dykes. This occurs largely independent of the type of terrane into which the dykes are intruded.

In addition, faulting, fracturing and shearing are less clearly manifested in the pattern of the aeromagnetic contour maps. 122

5. Structure and undifferentiated lineaments inferred from LANDSAT imagery and drainage patterns of the study area 5.1. Introduction

The maps of undifferentiated lineaments inferred from LANDSAT imagery and the drainage patterns of the study area are presented in this chapter. Brief observations intended to relate the lineaments and structural features (as referred to in previous chapters) ac­ company the maps.

LANDSAT lineaments have been taken as obvious topo­ graphical or geological alignments observed on LANDSAT images. Similarly, distinct alignments of stream courses observed on geological or topographical maps have been taken as drainage lineaments.

The maps of LANDSAT and drainage lineaments were originally prepared at a scale of 1 :250 000 -- the scale of both the LANDSAT images and geological and topographical maps -- and then reduced to a scale of 1:1 000 000, as presented in this chapter.

The following are the maps from which the drainage lineaments were inferred: 2228 Beit Bridge sheet, Geo­ logical Series, 1 :250 000, 1957; 2230 Messina sheet, Geological Series, 1 :250 000, 1981; 2326 Ellisras sheet, Geological Series, 1:250 000,1959; 2328 Pietersburg sheet, Geological Series, 1959; 2330 Tzaneen Topo­ Cadastral sheet, 1 :250 000; 2426 Thabazimbi sheet, Geo­ logical Series, 1:250 000, 1974; 2428 Nylstroom sheet, Geological Series, 1:250 000,1978. These maps are published by the South African Geological Survey. 123

The LANDSAT lineaments were taken from systematically corrected, edge enhanced (Band 5) images produced by the Satellite Remote Sensing Centre of the C.S.l.R., as follows: Scene ID~ 22420-07073, WRS: 181-75, date 07-sep-81 at 09h07, centre S21-42 E31-09; Scene 10: 22421-07132, WRS: -182-75, date 08-sep-81 at 9h13, centre S21-43 E29-43; Scene 10: 22384-07082, WRS: 181-7, date 02-aug-81 at 9h08, centre S23-06 E30-52; Scene 10: 30104-07180, WRS:182-76, date 17-jun-78 at 9h18, centre S22-59 E29-34; Scene 10: 22386-07195, WRS; 183-76, date·04-aug-81 at 9h19, centre S23-06 E27-59.

The strike orientations of the .LANDSAT and drainage lineaments were determined and classified into 30° intervals beginning from 0°= north. The length of the lineaments was measured within each class and then the sum of each class was expressed as a percentage of the whole; the results are represented in rose diagrams.

5.2. Description 5.2.1. Bande1ierkop domain

A large number of undifferentiated lineaments have been inferred from LANDSAT imagery and the drainage pattern of the Bande1ierkop domain (figures 57 and 58), many of which coincide with known faults, fractures, shear zones and dykes transecting the region (refer figures 11 and 12).

The strike orientations of the LANDSAT and drainage lineaments are represented in figures 56 and 59 res­ pectively. A northeast direction predominates in both 124

diagrams, therefore closely following the main direc­ tions of faulting- and dyking (figures 6 and 7).

No information as to the complex folding picture of the Bande1ierkop domain was gained from either the LANDSAT or drainage lineament patterns.

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180·

Figure 56. Rose diagram of LANDSAT lineaments - Bande1ierkop domain. (Total length of lineaments: 517 km; 0°_30°: 10%, 30°-60°: 31%, 60° 90°: 18%, 90°-120°: 14%, 120°• 150°: 16%,150°-180°: 11%) ·C.

BANDELIERKOP DOMAIN Scale 1: 1000000

·0 ...... --- ... .:

AREA 2 ~ -"" -

Figure 57 •. Map of undifferentiated lineaments inferred from lANDSAT imagery - Bandelierkop domain. (Base images: refer 5.1.) BANDELIERKOP DOMAIN Scale 1-1 000 000 .. / .. ___ I .... ~ /" .: /\ .>:"// /~J II '- ~ ./.>:...\' ...~ ~ / I- .. ::. ~ / ..... / /1 ..•. -.. 1':::::- --- -:•••••••• \ - I/// // - '<, .... I // - ../ ...."...: ../ ..\ .\.. \ \ / . .

AREA 2 - ~ ~. ~\\~~ '~( - /\ ~ -: --- / -----;/ // I. //~ -, --\

-- ~ .> --- '(1/ / /NJ-::- /~/ -- ./ /in- \ / /' - .,. - / \).. - /'/,--;-:-.-~---- \ - ~ / -: . 31° E

Figure 58. Map of undifferentiated lineaments inferred from the drainage pattern - Bandelierkop domain. (Base maps: refer 5.l.) 127

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Figure 59. Rose diagram of drainage lineaments - Bandelierkop do­ main. (Total length of lineaments: 1470 km; 0°-30°: 14%, 30°-60°: 36%, 60°-90°: 23%, 90°-120°: 12%, 120°• 150°: 11%,150°-180°: 5%)

5.2.2. Al1days domain

The undifferentiated lineaments inferred from the drainage pattern of the Al1days domain (figure 62) are oriented in two main directions, viz. east-northeast and north-northwest to north-northeast (figure 60).

The east-northeast oriented drainage lineaments tend to coincide with dykes, faults and fractures (see figures 17 and 18), whereas the lineaments of north- 128

northwest and north-northeast orientations would appear to reflect_the roughly north trending region- al fold structure, typical of the A11days domain. The latter structure is poorly displayed on LANDSAT images.

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Figure 60. Rose diagram of drainage lineaments - Alldays domain. (Total length of lineaments: 506 km; 0°-30°: 18%, 30°-60°: 4%, 60°-90°: 33%, 90°-120°: 5%, 120°-150°: 17%,150°-180°: 21%)

5.2.3. Southern Beit Bridge Complex domain

A strongly developed east-northeast direction pre­ dominates among the lineaments inferred from the LANDSAT 129

images of the eastern portion of the Southern Beit Bridge Complex do~ain* (figures 61 and 63). It re­ flects the structure of the SBBCd, where all the strata are strongly ali9ned in the regionally ex­ tensive east-northeast direction.

Only a small number of lineaments are inferred from the drainage pattern of the SBBCd (figure 62) and no rose diagram of orientations has been prepared. It is nevertheless clear that drain~ge lineaments of east-northeast orientation are also frequent.

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-+--t--t--+--+-~ 90·

Figure 61. Rose diagram of LANDSAT lineaments - Southern Beit Bridge Complex domain (eastern portion). (Total length of lineaments: 210 km; 0°_45°: 0%, 45°_60°: 16%, 60°_ 75°: 63%, 75°_90°: 14%, 90°_105°: 0%, 105°-120°: 2%, 120°_135°: 3%, 135°-150°: 0%, 150°-165°: 3%, 165°-180°: 0%)

* No data from the western portion of the SBBCd was available for this study . - ALLDAYS1and SOUTHERN BElT BRIDGE COMPLEX 2 DOMAINS -. ..-: ,.--: --~.-- _ ",,", ~ ....----- .. •• -" - ""- '\. SCALE 1: 1 000 000 \ -\~-"

Figure 62. Map of undifferentiated lineaments inferred from the drainage pattern - Alldays and Southern Beit Bridge Complex domains. (Base maps: refer 5.1.) 300E 31°E 22°5

--- - "....",-... '...... '''\.

SOUTHERN BElT BRIDGE COMPLEX DOMAIN (Eastern portion)

SCALE 1:1000000

23°5

Figure 63. Map of undifferentiated lineaments inferred from LANDSAT imagery - Southern Beit Bridge Com­ --' plex domain (eastern portion). (Base images: refer 5.1.) w --' 132

5,2.4, Western Transvaal domain

The only two drainage lineaments identified in this domain originate from two chief tributaries of the Limpopo rivers which display strongly linear courses. They are here referred to as the Lenkwane river linea­ ment and the Crocodile river lineament (1 and 2 res­ pectively in figure 64), and are also manifested as lineaments on ERTS imagery (S.Afr.geol.Surv., 1974). Both probably represent major fractures (refer 3.2.4.2.2_).

5.2.5. Soutpansberg domain

The maps of LANDSAT and drainage lineaments of the Sout­ pansberg domain are presented in figures 67 and 68 res­ pectively.

The rose diagram of lineament orientations (figures 65 and 66) reveal mainly a predominant e~st-northeast trend with fewer lineaments of west-northwest orientation. Both diagrams display essentially the same orientations, although some slight percentage differences between them exist.

The greater number of the LANDSAT and drainage linea­ ments originate from dykes, fractures and faults, where­ as few of them are not clearly correlated with any of the known structural features of the region. 26°E 27°E I I I . I I I 24°5 WESTERN TRANSVAAL DOMAIN

SCALE 1: 1000 000 .-: .. / / •. ..I ..../' ,--...",,­ ..~.. •.,

...1 ....J-I250 S

Figure 64. Map of lineaments inferred from the drainage pattern of the Western Transvaal do- , main. (1): Lenkwane river lineament; (2): Crocodile river lineament. (Base map: refer 5.1.)

-' ~ ""' TN 134

, Figure 65. Rose diagram of LANDSAT lineaments - Soutpansberg domain. (Total length of lineaments: 656 km; 0°_30°: 4%, 30°-60°: 2%, 60°_90°: 66%, 90°-120°: 22%, 120°- 150°: 5%, 150°-180°: 1%)

TN

Figure 66. Rose diagram of drainage lineaments - Soutpansberg do­ main. (Total length of lineaments: 540 km; 0°_30°: 3%, 30°-60°: 14%, 60°-90°: 45%, 90°-120°: 25%, 120°-150°: 2%, 150°-180°: 9%) 30°

SOUTPANSBERG DOMAIN

SCALE 1: 1000000

-- / ------23°

Figure 67. Map of undifferentiated lineaments inferred from LANDSAT imagery ~ Soutpansberg domain. {Base images: refer 5.1.} 30°

SOUTPANSBERG DOMAIN

.... SCALE 1:1000000 ------/- / - --- - :------I / -- ...... - ...... ) - - ~. ---- »>: \ / ~ ~ ~- -;+7 <,

Figure 68. Map of undifferentiated lineaments inferred from the drainage pattern - Soutpansberg domain. (Base maps: refer 5.l.) 137

5.2.6, Waterberg domain

The maps of LANDSAT and drainage lineaments of the Waterberg domain are presented in figures 71 and 72.

The rose diagram of LANDSAT lineaments (figure 69) shows a predominant west-northwest trend with fewer lineaments of northwest and east-northeast orienta­ tions. This pattern of orientations closely follows that of faults, fractures and dyke intrusions of this domain (figures 38 and 39). The LANDSAT lineament~ displayed in the extreme northeastern portion of the domain mostly originate from a,dyke swarm, whereas there is not equally good correspondence between lineaments and dykes or faults-fractures in the rest of the domain.

The ros~ diagram of drainage lineaments (figure 70) displays a predominant east-northeast trend, with fewer lineaments trending west-northwest, northwest and northeast.

In the northwestern part of the domain there is a fair correlation between dykes, fractures and faults (fig­ ures 38 and 39), and drainage lineaments (figure 72), whereas no clear correlation between the latter linea­ ~ents and "known structural features is apparent in the rest of the domain. 138

TN

180·

Figure 69. Rose diagram of LANDSAT lineaments - Waterberg domain. (Total length of lineaments: 250 ~; 0°-30°: 5%, 30°• 60°: 10%, 60°-90°: 18%, 90°-120°: 37%, 120° -150°: 19%, 150°-180°: 11%)

TN

1800

Figure 70. Rose diagram of drainage lineaments - Waterberg domain. (Total length of lineaments: 827 km; 0°_30°: 12%, 30°• 60°: 17%, 60°-90°: 34%, 90°-120°: 18%, 120°_150°: 13%, 150°-180°: 7%) 29°

WATERBERG DOMAIN

\ SCALE 1: 1000000 "-.-

I I I I {a} I I I IL _ (c) ; ,-~"d /~ : ~~- I (b) . \'-... ~~I~~ '10- "\ : ~ I --

Figure 71. (a) Map of undifferentiated lineaments inferred from LANDSAT imagery - Waterberg domain. (Base images: refer 5.1.); (b) "Linear structures (LANDSAT and aeromaqnetic survey)II (after 2428 Nylstroom sheet, Geological Series, 1:250 000, S.Afr.geol.Surv., 1978); (c) no data. 29°

WATERBERG DOMAIN

SCALE 1:1 000000 - -- ,. / --/ 1, ~-~ - / / ------7 r-;- f.--_- f- -/ - II .:» r-»: / / - / ...... ------_,'t::::::: ------::.1--...... __ ~ v , ---- <, / -- ~

Figure 72. Map of undifferentiated lineaments inferred from the drainage pattern - Waterberg domain. (Base maps: refer 5.1.) , 141

5.7.7. Lebombo domain

The LANDSAT lineaments of the Lebombo domain (figure 74) closely reflect the structtire of the region, that is, a monocline striking roughly north-south (refer 3.3.3.). Most LANDSAT lineaments trend north-northwesterly (figure 73), with a secondary direction trending north­ west to west-northwest which correlates with faults and dyke intrusions.

A small number of drainage lineaments that are evident in the northern portion of the Lebombo domain (figure 75) originate from faults and fractures, whereas the northeast- and northwest trending lineaments encounter­ ed in the southern portion of the domain have not been clearly correlated with known geological or structural features.

TN

180 0

Figure 73. Rose diagram of LANDSAT lineaments - Lebombo domain. (Total length of lineaments: 244 km; 0°_30°: 2%, 30°_ 90°: 0%, 90°-120": 17%, 120°-150°: 22%, 150°-180°: 60°) 142 r------...L-..------22°S

- ..

LEBOMBO DOMAIN

23°30' SCALE 1:1 000 a00

Figure 74. Map of undifferentiated lineaments inferred from LANDSAT imagery - Lebombo domain. (Base images: refer 5.1.) 143 r------.&...------22°S

- ..

LE BOMBO DOMAIN

23°30' SCALE 1:1 000000

Figure 75. Map of undifferentiated lineaments inferred from drainage pattern - Lebombo domain. (Base maps: refer 5.1 .) 144

5.3. Interpretation

The lineaments inferred from LANDSAT imagery and from the drainage patterns of the study area tend to re­ flect only late structures such as fractures,· faults and dyke intrusions, hence "it i~ clear that th~y fail to be of use in the interpretation of the complex structure of the metamorphic Precambrian terranes of the northern Transvaal. 145

6. Synopsis and conclusions

The deformation history of the study area produced a wide range of structural styles and orientations on the rock units, as the study area comprises portions of the Limpopo Belt and the Kaapvaal craton, as well as younger cover terranes. Marked structural differences are recognized within each of these terranes.

The Southern Marginal Zone of the Limpopo Belt and the northern part of the Kaapvaal craton comprise areas 1 and 2 (Bdl and Bd2) r esoec t tvel yor the Bandel ierkop domain. It was emphasized that a remarkable contrast exists between the highly disrupted supracrustal xeno­ 1iths in Bd l , and the much bigger xenol iths of supra­ crustal rocks in Bd2. Although the high-grade meta­ morphic rocks of Bd1 are thought to be part of the Kaapvaa1craton's lower crustal rocks brought to sur­ face as a result of compression about 2600 Ma ago, the mechanism of uplift of these rocks is not fully understood.

In the light of the metamorphic transition known to exist between Bdl and Bd2, and of previous workers' interpretations of geophysical (gravity) investigation in the region, a model for the uplift of these rocks was proposed in this thesis. In terms of this model, the granulite facies rocks of Bdl are considered to have been upthrust roughly from south to north along one or more south-dipping listric thrust faults soling into a gently-dipping to flat-lying basal shear zone (figure gal.

Since this model reveals the existence of the Kaapvaal 146

craton partly on edge, the progressively higher grade of regional metamorphism (from greenschist to granulite facies) encountered in the Bandel ierkop domain' -- from south (Kaapvaal craton) to .north (Southern Marginal Zone of the Limpopo Belt)'-- could express the transi­ tion from upper crust to lower crust, as exposed on the present land surface.

It was shown that the Southern Beit Bridge Complex domain (SBBCd), which occupies a relatively narrow east-northeast oriented strip along the southernmost portion of the Central Zone of the Limpopo Belt, has a relatively higher strain than the surrounding Pre­ cambrian terranes. This evidence of heterogeneous strain was attributed, in the relevant chapter, to the presence of a shear zone along the SBBCd.

Although doubt was expressed as to the type of shear­ ing movement that took place along this shear zone (i.e. wrench or thrust type), it is now evident that, in accordance with the thrust model referred to above, the SBBCd can be properly regarded as the plane of thrusting.

In view of the steeply-dipping, south-southwest oriented fold axes obtained by Horrocks (1981) in an area south of Messina, it would appear that, at least in the area studied by him, the thrust movement would have been predominantly towards the north-northeast.

In terms of the same model, the regional fold pattern of the Alldays domain was regarded as having been formed by large sheath folds.

The fact that significant amounts of horizontal dis- 147

p1acement,perhaps hundreds of kilometres, may have taken place along the Central Zone - Kaapvaal craton boundary (e.g~ Coward, 1976; Coward Et 81., 1976; 8arton and Key, 1981; Barton, 1983b) 1S not disregard­ ed in this study. In view of the two events of de­ formation found in the Palala shear zone (McCourt, 1983) marking part of the southern boundary of the SBBCd -- one about 2800 Ma ago, and the other about 1850 Ma ago -- they may reasonably be considered to express the occurrence of two events of shear movement along that boundary, of which the older one could represent that of the thrust type favoured in the model referred to above, whereas a complicated late history of this shear zone may have involved one or more episodes of wrench-type movement.

It was suggested that a linked fault system -- charac­ terized by south-dipping, normal listric master faults soling into a gently-dipping major detachment surface (figure 9b) -- could explain the overall structure of the Soutpansberg domain. It was further envisaged that the systeo which developed earlier in the geo­ logical history of the region along the Central Zone ­ Kaapvaal craton boundary, and which is characterized by south-dipping thrust faults soling into a basal shear zone, could have controlled the late extensional displacement.

This study would therefore tend to show the importance of structures formed very early in the history of the earth. In the study area, these structures were shown to have controlled later events of deformation -- e.g. wrench and normal faulting, fracturing and dyking -­ all related to the fundamental geological grain formed about 2600 Ma ago. 148

From the qualitative interpretation of .the aeromag­ netic contour. maps of the study area t the impression was gained that most of the lineaments identified on these maps can be correlated with dyke intrusions.

On the other hand, the lineaments inferred from LANDSAT imagery and from the drainage patterns of the study area tend to reflect fractures t faults and dyke in­ trusions.

Hence it is clear that b~th lineaments referred to above fail to be of use in the interpretation of the complex structure of the metamorphic Precambrian terranes of the northern Transvaal. Instead t they may be of value in future research into the later episodes of deformation in the study area, such as faulting, fracturing and dyking. 149

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